Simply put, a painting is a bunch of different colored shapes. The configuration on these colors have been discussed here and the placement of the shapes have been discussed here. The only significant thing left to discus is the edges of those shapes.
Consider this: in Rembrandt’s time, if an artist wanted to evaluate the lightness values of a painting distinct to its color component, the only option was to view the painting in candlelight. The dull light leeched most of the color values from the painting.
I have talked about how you might think about the color values of a painting. I have talked about what paints you might use to paint a painting. Now I am going to talk about how you might use them. I shall do this by addressing one, simple term: contrast. Contrast is a simple word to say, but a most complex beast to define.
Representational painting requires that the illusion of depth be wrought upon a flat surface. This illusion is not accidental and is effected through a ‘box of tricks’ which are one of the most important tool sets of a painter.
One of the crucial differences between a drawing and a painting is that a painting usually requires more careful consideration of its 2D spatial qualities. In a drawing we might carelessly place a rendering of a figure upon the paper and give no consideration at all to the surrounding white space that extends to the edges and the four corners. In a painting this is less likely to be true. This consideration is called composition. It may be considered to be a conscious organizing of its many spatial atributes.