Paint is paint. You can paint a wall, paint a sign, paint a car. The majority of the time, paint is used to cover a thing with a flat color. But like I said earlier, you can also use paint to create the effect of chiaroscuro, which is the illusion of light on form, and creating this effect is very logical. Now, I want to state here that while creating the illusion of chiaroscuro is very straightforward and frankly mathematical and can be expressed and understood fairly quickly and easily, truly understanding the "art" of painting is a personal journey that will take sometimes more than the artists' lifetime to master. That's how Vincent can say "painting is like algebra" and at the same time also speak about it's mysteries...
So back to the logical side of things. As I said in my earlier post, the effect of chiaroscuro is about painting two different "keys" of the same relationship of local colors, one light in value and one dark in value. What is interesting is how flexible the artist can be in expressing the "feeling" of a certain local color relationship. For example the "feeling" of the difference in value and color between, say, a white shirt, a skin tone, a red vest, and blue jeans, can be all over the place in terms of how light or dark or even overall hue (blue or orange or any color) I tried to make a new chart that further clarifies my thoughts on the idea. Sometimes images succeed when words fail.