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This question doesnt relate only to yellow, but basicly to all colors.

Lets say:

I have pure Yellow painting color and also Yellow painting color mixed from Red and Green.

If I would turn on pure Yellow light(Total absence of any other light colors) in my room, would both painting colors reflect the light, or only the pure painting color ?

I am asking because I cant reproduce the experiment home

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    $\begingroup$ Somewhat beside the point, but you can make yellow light by mixing red and green, but not yellow paint by mixing red and green paint $\endgroup$
    – doetoe
    Feb 18, 2019 at 21:27

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As a comment points out, you can hardly make yellow painting by mixing others. Yellow is said to be a primary colour of ink. You should have chosen another one, like green, which can be made up by mixing blue and yellow, as everybody knows since 3 years old haha.

So, if you don't mind, I'll talk about this green case. You can apply it to any colour.

So you have those two samples: pure green ink, and mixed yellow-blue=green. Let's suppose they are ideal inks, that is, they only reflect their colour line.

If you poured pure green light on them, the first one would be visible and would appear green. The mixed one would not be visible, it'd seem black.

That's obvious, because the green ink can reflect green, but a mixture of blue and yellow can reflect either yellow or blue (or both at the same time), but not green.

However, if you shed a light made of pure yellwo light + pure blue light, then the mixture would be fully visible (as all of its components would be reflecting), and the pure green ink would seem black.

This shows the effect of metamerism. Some samples can seem the same under a light spectrum and become different under a different lamp.

But is it so clear? No. Now let's tell the truth: nothing is ideal. In the real world, all inks have a curve of reflectance. This means that real paint tones do not reflect one single wavelenght, but rather an interval of wavelenghts. It's usually a maximum at the colour's centre and then it goes down to zero, but there are many kinds of spectra. Have a search out there.

And this implies that, when you pour some light (which is not pure either), all samples will partially reflect some of it. So it is much more likely that you see oine darker, but not black.

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  • $\begingroup$ Thanks for clear explanation and for pointing out impossibility of making yellow paint. Didnt know about that $\endgroup$
    – Martin
    Feb 18, 2019 at 22:06
  • $\begingroup$ "That's obvious, because the green ink can reflect green, but a mixture of blue and yellow can reflect either yellow or blue (or both at the same time), but not green." Unfortunately this just isn't true. Most Yellow and Blue pigments actually do reflect green wavelengths. They also reflect other wavelengths, such that our eye perceives them to be Yellow or Blue. When the two pigments are mixed, the resulting pigment reflects mostly green wavelengths, as the Blue pigment absorbs yellow/red wavelengths, and the Yellow pigment absorbs blue wavelengths. $\endgroup$
    – Sean Skelly
    Apr 22, 2020 at 23:26

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