Here comes the first, the most important rule of painting: light is the only thing we can see. It's not an object, not a color, not a perspective, not a shape. We can see only light rays, reflected from a surface, disturbed by the properties of the surface and our eyes. The final image in our head, one frame of the never-ending video, is a set of all the rays hitting our retina at that one moment. This image can be disturbed by differences between the properties of every ray—every one of them comes from a different direction, distance, and they may have hit a lot of objects before hitting your eye last. That's exactly what we're doing when painting—we imitate rays hitting different surfaces (color, consistency, gloss), the distance between them (the amount of diffuse color, contrast, edges, perspective), and most certainly we don't draw things that don't reflect or emit anything to our eyes. If you "add light" after the picture is almost done, you're doing it wrong—everything on your painting is light.
