When dealing with light, is it true that a surface will only reflect the colour that the light emits? E.g. if you have a red light, but a violet surface, then the amount of light reflected from the surface is how much blue is in the violet, and all colours other than blue do not get reflected. When working with RGB(A) and blending two colours (e.g. a light diffuse colour with the material's diffuse colour) it is generally accepted that this can be performed by multiplying each colour's channels together. For example: Red = light.R * diffuse.R; Green = light.G * diffuse.G; Blue = light.B * diffuse.B; This works fine if the light source is Red, Green, Blue, or white. For example, if a red light is shinning on a yellow surface, all channels other than red would be multiplied by 0 therefore only the intensity of the red within the yellow is reflected. Therefore the light is effectively filtering out all colours but red. But if the situation is reversed and the light is of a colour that must be represented by multiple colour channels (e.g. yellow), then both red and blue will be reflected, when I thought all colours except yellow were suppose to be filtered out (thus only reflecting the magnitude of yellow in the colour). I'll try to explain it the best I can mathematically of the formula I'm thinking of. Let's say we map the colour (the RG channels in our example of yellow light) being mapped as a spacial vector (XY). The colour red can be represented on this scale as {1,0) (X being red, Y being green), and yellow can be represented as {0.7, 0.7}. To find the amount of yellowness in the red we rotate both vectors 45* clockwise so that yellow is now {1,0} and red is {0.7, -0.7}, so now X represents an imaginary yellow channel. When we multiply these two vectors together the output returns {0.7, 0.0} so the red colour has a magnitude of 0.7 in the direction of the yellow channel. Now multiply this magnitude by the RGB of the yellow colour (0.7 * {0.7,0.7,0}) returns the final output colour of {0.49, 0.49, 0}. There problem I see of this method of calculating light is that only one colour (in various shades) is actually reflected, therefore a white light would reflect everything in grayscale. I'm thinking the method that I'm suggesting won't work with RGB colour, instead the colour could be converted to HSL (but representing the hue as a normalised RGB colour) with the saturation representing how much of the other colour channels will be filtered out. Thoughts?

If red, blue and green were single frequencies, the "generally accepted" model would be correct. The problem is that there is a continuum of frequencies, each of which has contributions to R, G and B (some of which may be effectively 0), and the three numbers you end up with are only integrated values which do not contain all the information.

For instance, you could pick two frequencies that are perceived as red only and as green only. Consider a yellow light source Y1 which has a spectrum where only those two frequencies are present. If you consider all the frequencies in between those two, you'll find one where the contribution to red and green is equal. Consider a yellow light source Y2 which has a spectrum where only that one frequency is present. If you now illuminate a red object and a green object with Y1 you could get to see both colors just fine, but it could be the case that when you use Y2 on those two objects you actually see them both black. Or they could both look yellow when illuminated with Y2. The result depends on what the reflectance spectrum of the objects looks like.

Modeling this accurately doesn't seem feasible in the context of game graphics. The generally accepted "there are only three frequencies" model is probably good enough.