Shaders means that you can modify the colors ingame, like for example when you use eagle vision in assassin’s creed. You can also use shaders for visual effects like explosions, water ripple, whatever. For example, iof you have blue water, but a shader adds dynamic white foam and dark blue depth, that still counts as one color.
Light can be anything, yeah, it doesn’t matter if it brightens or darkens a color, it still counts as one.