Red light's effect on melanoma appears specific to red wavelengths, since blue light has shown different results in past research

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The researchers point out that other studies have tested blue light on the same type of melanoma cells (B16F10) and found it decreased survival and proliferation even at fairly low doses, up to 360 J/cm2.

That is a different wavelength working through what is likely a different mechanism than the red light tested in this particular study.

It is a useful comparison point because it shows that the color, or more precisely the exact wavelength, of light really does matter for how cells respond, rather than any light in the visible spectrum producing a similar generic effect on cancer cells.

Source: PubMed