Think of it this way: the color channel represents the diffuse color of the object (Cinema’s diffuse channel is something else). The brightness of the color is affected by lights. The luminance channel, OTOH, applies color to your object that is NOT affected by lights. It also generates light if you are using global illumination.
So if you have both color and luminance they’re going to blend together and the luminance is going to predominate because it’s at full strength regardless of scene lighting.
What is it that you’re trying to achieve?