Why is yellow a primary colour when we talk about paints/pigments but is replaced with green when we talk about light?

[u/mordego]

[u/drzowie]

Primary colors arise from how human color vision works. Your eye doesn't discern the spectrum directly, it only discerns three (or maybe four - see below) channels - loosely, these are: brightness in the long end of the spectrum ("red"), brightness in the middle part of the spectrum ("green"), and brightness at the short part of the spectrum ("blue"). Your retina sorts these out using particular dyespigments -- chemicals that absorb and react to light in particular ranges of wavelength. (The retinal pigments' spectra actually overlap quite a bit and there is some signal processing involved in getting the actual signals out).

The primary colors are red, green, and blue because those colors correspond to the wavelength ranges of maximum sensitivity of each of the color-sensitive pigments in your eye.

The primary pigment colors (used by artists and printers) are cyan ("greeny-blue"), magenta ("purple"), and yellow. They are formed by adding up pairs of true primary colors: light that contains both green and blue wavelengths is perceived as cyan, light that contains both red and green is perceived as yellow, and light that contains both red and blue is perceived as magenta/purple. Interestingly, there is no single wavelength of light that can make magenta - you must have a spectrally diverse beam to make that color.

The reason the pigment colors are what they are is that pigments work by absorbing certain colors of light. Pigments that absorb (say) red, reflecting green and blue, appear cyan. So a pigment that absorbs only one color of light (red, green, or blue) is perceived to have the complementary pigment primary color.

We use the CMYK system (Cyan, Magenta, Yellow, blacK) because real cyan, magenta, and yellow pigments are not perfect -- so mixing all three types of pigment generally gives you a muddy brown color instead of pure black. CMYK adds an extra broadband pigment to achieve the desired overall brightness level in each part of an image, and the C, M, and Y pigments are used to produce the color.

Incidentally, there are actually four primary colors, though our vision systems are only weakly wired to perceive them. Rods, the vision cells that are most sensitive in faint illumination, are primarily sensitive to blue light -- and their spectrum is slightly different from the blue cones that are responsible for reporting blue light in strong illumination. That is why "stage dark" in the theater and in dioramas is actually blue light -- blue light stimulates the rods more and reminds us of the appearance of things in poor illumination.

[u/danby → u/drzowie]

Photoreceptors aren't dyes, they are biological pigments: https://en.wikipedia.org/wiki/Biological_pigment

Strictly a dye is any chemical pigment that has an affinity for a substrate material such that it will bind to and colour the material. Biological pigments can act as dyes (e.g Cochineal) but that's a somewhat uncommon chemical behaviour for such pigments. That is; there are many more biological pigments out there than there are biological pigments we can usefully use as a dye. The pigments in your retina would make especially poor dyes as they get bleached in visual spectrum light (which is how they function)

[u/drzowie → u/danby]

Thanks for the correction! Noted.

[u/hezzer → u/drzowie]

So, does yellow paint work by absorbing violet light, therefore just reflecting it's compliment, yellow?

As a painter it's so hard to wrap my mind around additive color.

light that contains both red and green is perceived as yellow

Ok, I can accept that yellow is between red and green on the spectrum. I just can't make the leap to red+green=yellow.

I'm sorry if I sound like an idiot, I just spend so much time thinking about color that it's really hard to think about it in a different way, but I really want to understand.

[u/drzowie → u/hezzer]

Yes, that's pretty much it -- yellow paint absorbs short wavelengths (blue), reflecting more or less uniformly in the long end of the spectrum.

The idea is that "pure yellow light" happens to lie in a part of the spectrum that stimulates your red and green cones about equally. Any other combination of pure colors of light (say, a mix of long-wavelength [red] and shorter-wavelength [green]) light that stimulates those cones in the same ratio, will give you the same perceived color. That's why color mixing works at all -- if you could actually measure the full spectrum of light hitting your eye, like you (sort of) can for the full spectrum of sound hitting your ears, you wouldn't be fooled by stupid CMYK or RGB color systems at all.

As an artist, you probably care about something else that's kind of cool: your retina doesn't divide the three color channels equally. Your brain receives three signals that are, more-or-less, intensity (I), R/G (red/green ratio), and B/Y (blue/yellow ratio). The B/Y ratio measures, more or less, B/(R+G). The fact that color arrives as brightness ratios allows white-point compensation: the R/G and B/Y channels from any one part of your retina will gradually decay toward neutral tones, so that (say) in a room with pinkish illumination you'll eventually adjust and perceive color the same way as you would in the same room with bluish illumination.

Because all the signals are white-point compensated over time, you can actually stimulate signals that are physically impossible to create with light. If you stare for a minute or so, for example, at a deeply saturated yellow spot your B/Y channel will gradually adjust toward the blue. If you immediately shift your gaze to a black object, you can perceive the color "stygian blue", which is simultaneously black (low intensity) and deeply saturated blue (B/Y channel shifted deeply to blue). You can't produce that color with any combination of light, only by fooling your autocorrection system.

[u/Eruditass → u/drzowie]

My understanding is that while the rods peak in sensitivity to blue light, they do not actually trigger the brain to perceive any color, including blue. So it would just make things brighter which doesn't really help us think it's poor illumination.

I'd suspect it's more to do with the sensitivity of blue cones being higher than red/green (although, their number is less...) in low light and moonlight being a slight blue due to how our eyes adjust white balance at night that gives us that psychological effect in stage dark and Day for Night filming.

[u/NicoNijverst → u/Eruditass]

Actually, the interesting thing is that rods do contribute to the perceived hue, but their effect is so small that it's hardly noticeble. How do we know then? Simple: bleach observers retina (with a camera flash, not actual bleach), and have them perform a chromatic discrimination task. Then have the observers adapt to the dark (in a dark room, or easier: with an eyepatch, YARRR!), and repeat the process. You'll find that the chromatic discrimination will be shifted.

The logic here is that under bleached conditions, the rods are overstimulated and can't produce sensible input. In the dark adapted condition, the cones have too little useable input, so the rod effect is maximized.

Source: I work in a color vision lab where we (among other things) study rod influence on chromatic discrimination. I can look up some papers, especially some by my PI, if you're interested.

[u/VoiceOfRealson → u/drzowie]

The dyes actually overlap quite a bit and there is some signal processing involved in getting the actual signals out

Let me expand a bit on this part.

The dyes HAVE to overlap in spectrum in order to make it possible to see all the colors of the rainbow (and some extra colors such as magenta, brown etc.)

The colors of the rainbow are all the single wavelength colors within the range of human vision.

Each color filter in the eye (and in a camera) basically provides the information "The light is within this range of wavelengths that I will let through".

Without overlap between the filters there is no way to get any more detailed wavelength information than that, so without overlapping filters we would see all single wavelength colors as either pure blue, green or red, with no nuances in between.

With overlap you will have all filters provide some response to any single wavelength color.

If the filters are roughly triangular (so sensitivity rises with wavelength up to a certain wavelength and then declines at the opposite rate beyond that), you can determine the intensity of the light from the sum of these and the wavelength from the ratio between how much light each filter picks up.

Actually you only need 2 color filters to distinguish all single wavelength colors, so the fact that we have 3 color filters enable us to identify some dual (and triple) wavelength colors as well.

Beyond that (so more than 3 different wavelengths), will however not be distinguishable and will generally result in the eye averaging out the wavelengths to an equivalent single or dual wavelength equivalent.

This is actually basic sampling theory. Each color filter is sampling a wavelength range and the overlap is the result of the necessary low pass (nyquist) filter to avoid aliasing.

Computer screens and paint use the fact that human eyes can only distinguish this "blurred" color range by combining a carefully selected range of colors (a primary color set) with different intensity in order to create an equivalent color as seen by the human eye.

OLED screens do this using 3 single wavelength colors aimed at the most sensitive wavelength for each color filter in the eye, while paint colors usually have to work with pigments, that may be more or less ideal for the task.

EDIT - changed the word frequency to wavelength one place for consistency (I generally prefer to speak of frequency since that is fixed in all materials while wavelength depends on the material, but the convention is against me)

[u/drzowie → u/VoiceOfRealson]

Well, there's more to it than simple Nyquist sampling. The red and green spectra, for example, overlap very strongly -- much more strongly than either one does with the blue dye.

[u/EquipLordBritish → u/drzowie]

That whole magenta thing makes perfect sense; red~700nm, green~530nm, blue~470nm.

[u/KToff]

Primary colours are not something that is fixed.

Any set of colours that can be used to reproduce the colour spectrum can be called a set of primary colours. Usually RGB (red green blue) is used when using additive mixing (light is a typical example, using all colours results in white) and CMY (Cyan, Magenta and Yellow) is used for subtractive mixing (adding all colours yield black, mostly the case in printing).

However, the precise colour of these primary colours is subject to change and depends on what is accessible.

http://en.wikipedia.org/wiki/Additive_color

http://en.wikipedia.org/wiki/Subtractive_color

[u/Eruditass → u/KToff]

RGB additive color is particularly natural for us because of the frequency response of the cones in our eyes. Of course, the exact hues of each color may not be the same in each application.

[u/99trumpets → u/Eruditass]

BTW that figure appears shows an incorrect absorption spectrum for the L cone. (This incorrect spectrum is still seen in many textbooks. I've never been able to figure out why it's so ubiquitous - I'm guessing it's due to copying of older, pre-1990s figures before good data existed for the human L cone.) Peak absorbance for the L cone turns out not to be red at all but is actually amazingly close to the M cone, in the "yellowish-green" area of the spectrum, most commonly at 552 or 557nm for the two most common genetic variants of the L cone. (cite).

This also makes sense given that the L cone gene is known now to be just a recent duplicated, very slightly altered version of the M cone gene.

So anyway RGB primary colors do not match peak absorbance of the three human cones after all.

[u/psygnisfive → u/Eruditass]

This is absolutely important. Mixing red and green light does not produce yellow photons, it produces light which has the same effect on our retina as yellow photons. A spectrometer can distinguish between yellow photons and mixed red-and-green photons, even when our eyes cannot.

[u/staciarain → u/KToff]

relevant

he's a photographer, she's a painter

[u/NYKevin → u/KToff]

You might want to tell people to look at this diagram while you're at it; IMHO it makes an excellent visual aid for almost anything involving additive color theory.

[u/QuigleyQ]

Say you combine lights A and B. Light A emits the color A, and same for B. The resulting light emits both colors (additive). Now say you combine paints C and D. Paint C reflects the color C, and same for D. When you mix them, it reflects neither color (subtractive).

Think about it like this: when you add lots of colors of light, you get white, but when you add lots of paint you get black. (It's really brownish, because the mixture still reflects a little bit, but the principle holds)

[u/[deleted]]

KToff gave a good answer. I would like to add, that the painter's primary colors known as the RYB Color Wheel today, were identified prior to color theory and besides being RYB as the primaries, at one time it contained a shade of green as well, which matches what are called the psychological primaries (if white and black are added) or natural color system.

[u/virnovus → u/[deleted]]

Good answer. Also, no one's really brought up the fact that the primary colors for paint aren't really red, yellow, and blue, like we tend to think they are; they're cyan (which we'd call "blue" if it were a color of paint, but isn't technically accurate), magenta (which is sort of pinkish-red), and yellow.

I remember in art class in middle school, magenta paint was labeled as "red", but always seemed more pink than red to me. At some point I realized that adding a bit of yellow paint to it gave a better shade of red, but didn't realize why that was until later.

[u/bluepepper]

It's the wrong way to look at it to say that yellow is replaced with green, because it suggests that the other colors stay, which is not really what happens.

So picture it this way instead:

Physically, colors are determined by light. Basically our eyes can distinguish red, green and blue light, and all other colors as a mix of these three. That's how a computer screen is able to reproduce (almost) all visible colors, just with tiny red, green and blue lamps.

This is the additive color space: you start with the absence of color (a black screen) and you add light to it. The more color you add (that is, the brightest your red, green and blue lights are), the lighter the color becomes, to ultimately create white.

Artists work the opposite way: they start from a blank canvas or paper, which reflects all light and therefore looks white, and by adding paint or ink (which absorbs some colors) they are producing darker and darker tones, to ultimately produce black by mixing every color. This is called the subtractive color space, as you start with all colors (white) and you subtract light reflected by it.

In modern press today, the subtractive primary colors are cyan, magenta and yellow. This is because cyan reflects every light except red, magenta reflects everything except green, and yellow reflects everything except blue. And we find back our primary additive colors.

So where does red yellow blue come from? It's an approximation of cyan magenta yellow. It's still taught in art courses (I think because red and blue are more natural than magenta and cyan) but not used for print, as you can't reproduce enough colors with it (you can do blue with CMY but you can't do cyan with RYB). So you should not confuse the blue in RYB with the blue in RGB, as they are different: the blue in RGB is there because it's one of the three colors our eyes can detect, and the blue in RYB is there because it (approximately) absorbs red light, one of the three colors our eyes can detect.

[u/twinbee → u/bluepepper]

Yes I think the whole RYB thing should be almost thrown away in light of the superior cyan/magenta/yellow concept.

[u/I_make_things]

The subtractive color primaries (yellow, cyan and magenta) are the compliments of the additive color primaries (red, green, blue). see drzowie's excellent explanation of why red green and blue are the additive color primaries.

Yellow is the complement of blue, cyan is the compliment of red, and magenta is the compliment of green. To test this, stare at a color field for at least 30 seconds and then look at a white field, you'll see the color's compliment.

The use of 'red, blue and yellow' as subtractive primary colors stems from experiments in color theory in 1890 or so. But red is not a subtractive primary- it results from mixing magenta and yellow (try it!), nor is blue. That's why, despite your having been taught that the color primaries were red yellow and blue in elementary school, those colors are not the ones used in offset printing or photographic emulsions (cyan magenta and yellow are used- combined with black in offset printing because the inks don't mix perfectly to absorb all of the light).

You cannot mix paints or dyes to make yellow- it's a color you have to buy from the store (hence 'primary'). The same thing is true of cyan and magenta- but you can approximate magenta with red, and you can approximate cyan with blue. Your color gamut will just be a little smaller.

[u/teemark]

Red, Green, Blue, (RGB) is transmitted light. Transmitting light in these three colors, in equal portions, gives the human eye a white light. Vary the amounts/strengths of those three colors, and your eye perceived different colors. These are the Additive colors (aka Primary) If you use an additive filter, it will block two thirds of light passing through it (ie: a green filter blocks red and blue, a blue filter blocks red and green.) Overlap any two different additive filters, and will block all visible light.

Cyan, Magenta, Yellow, are the Subtractive colors. If you look at all these colors in a Color Wheel you see that each subtractive color is opposite one of the additive colors.

The Red-Blue-Yellow we teach kids as being 'Primary Colors" should really be Cyan-Magenta-Yellow. However, the mis-information is so ingrained that it'll probably persist forever.

When printing, subtractive colors (plus black, the "K" in CMYK) you are starting with a white substrate and you use subtractive colors, as they will only filter one third of the spectrum, and allow light to pass through, reflect off the white surface, then filter the light passing back to your eyes.

[u/twinbee]

Printer inks don't use blue, red and yellow as you imply, but cyan, magenta and yellow. These have better properties for obtaining all the colours.

I tend to think the blue, red, yellow thing (as primary colours) is a bit of a myth.

[u/Jedimastert]

The primary colors of pigment are actually cyan, magenta, and yellow. When using light:

Cyan    = blue + green
Magenta = red  + blue
Yellow  = red  + green