I am constantly amazed that people don't know the most basic premise of reflection.

If the color of arriving light is L

and the color you see on an object lit by L is S

and the color of the object itself is C

then the relationship is:

S = L * C

That means that what you See (S) is the result of multiplication (*) of the light color (L) and the object color (C).

If you see yellow (S = yellow)

and the light arriving is white (L = white)

then the color of the object must be yellow (C = yellow)

because the only way you can solve this:

yellow = white * C

is

C = yellow / white = yellow

Gray objects are objects where the RGB reflection values of its color (C) are equal.

So:

S = L * C

Break that down, assuming k is the equal RGB gray level found in C:

S = L * [k, k, k]

And break the light down into lr, lg, lb:

S = [lr, lg, lb] * [k, k, k]

The product is:

S = [lr * k, lg * k, lb * k]

I hope you can grasp the meaning of that. Suppose it is a 50% gray, so that k = .5.

Then what you see is:

S = [lr * .5, lg * .5, lb * .5]

Another way to put it is:

S = L * .5

Quite simply, a 50% gray object reflects exactly the hue and saturation that is striking it, but only half the value or intensity or brightness, whatever word you like.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)