You are thinking of a grey card: https://en.wikipedia.org/wiki/Gray_card https://en.wikipedia.org/wiki/Gray_card However color matching is more complex than th

You are thinking of a grey card: https://en.wikipedia.org/wiki/Gray_card https://en.wikipedia.org/wiki/Gray_card However color matching is more complex than that. "Color of the light source" varies considerably, not only due to the illuminant itself, but also the materials the illuminant is reflecting off of (e.g. walls). And color temperature only defines black bodies, which most light sources are not: you need information about the full spectrum of the illuminant to determine how it interacts with the material to be color-matched. This is getting a bit into metamerism: https://en.wikipedia.org/wiki/Metamerism_(color) https://en.wikipedia.org/wiki/Metamerism_(color) (Of course, paint cannot match an arbitrary spectrum. Matching under specific lighting conditions is the least you can hope for.) And the above is considering color of a surface only as a scalar value, but it's really a function of illumination angle as well, due to gloss. Direct lighting on a glossy surface reflects more of the color of the gloss, whereas oblique lighting reflects than the paint itself. Diffuse lighting is some combination thereof. (I am not a color scientist by any means, just interested in the topic. Experts please correct!)