The color that we see on a painted surface is reflected light. The paint reflects the light that we see and call it's color, but it is actually sucking up all the colors we don't see. For instance red paint is actually yellow and blue. "Red" paint absorbs and gathers blue and yellow light and REJECTS the red light, it is reflected back at us to perceive as "red".
Get it? I didn't think so.
White light is all the colors of the spectrum mixed together, but a white canvas is actually black because it has no color of it's own. It reflects all colors back to us and we only perceive it as white. When I begin to paint on a white panel or canvas I am merely stripping away or erasing the layers of light and revealing the art that was unseen. I am taking colors away from the white, so that only some colors are reflected back and no longer perceived as white.