Well to an artist, anything that changes the image they're working on is a color. Sometimes they'll start with all white, paint it all green, then paint white over it. White is therefore a color to work with. If they always started with black, and never painted with black it wouldn't be a color.
It's like dark is just the absence of light. Black is just the absence of color, which in itself can be considered a "color."
THE JACKEL