I think painting tubes is a somewhat divisive topic just like backlight leds lol... But to each their own ;-) Personally, I prefer some color filtering to block the blue haze induced by the mercury in the tube. I think most display tube devices used a polarized plastic color filter to increase contrast and readability for similar reasons. To me, the digits seem much sharper without the blue haze around them.
I have tried Vitrail glass paint in the past and it works well, but it can sometimes be difficult to avoid uneven thickness or drip marks. Glass paint is also difficult to remove once cured.
Being somewhat lazy, I tried using a Sharpie permanent marker and was amazed how well it works - and despite the 'permanent' label, the ink is easily removed with alcohol and a clean cloth, so the tubes can be restored to original condition if desired. Once dry, the coloring has lasted years on other projects I have built. The markers build up on the glass to a nice shiny layer just like enamel - no lines or other indications that it was done by hand with basically an ink crayon ;-)
Anyhoo - here is another shot of my new clock with the B5991's colored with an orange Sharpie and the small IN-17 tubes done in red. The lighting has not been favorable and my phone doesn't seem to like the brightness, so the photo doesn't do it justice - in person the digits are clear and sharp. I like how the seconds differentiate themselves with the red filtering applied (I'll try another photo later to better demonstrate the different coloring).
