I've just noticed this, and I wanted to share. I was sorting out the pictures that my daughter made over the last couple of years and I could clearly see that there were several different phases of coloring skills development. This is based just on my observations of my own child. She never had art lessons.
1) If you give a coloring book to a toddler, he'll be doodling on it as if the list is blank. The fact that the crayon leaves a colored line is too fascinating for a child and he is too occupied trying to figure it out so he does not notice the image on the paper, he would not care to color it.
3) The child colors the whole image with the crayon just to cover it, he is not even trying to stay in lines. Covered = Colored.
(image on the left = 4 yo)
(image on the left = 4.5 yo)
7) The child tries to make colors true-to-life. I don't think that, unless the child is taught so, he'll ever attempt to make images look 3 dimensional (by adding shades, shadows, using mixed colors -like different shades of yellow, red and brown to color a leaf).
Do you pay attention to how your kids color/draw? I would like to hear what other people think about my theory. I'm sure that professionals like OTs and art teachers can explain why exactly the child does what he does. And I know that the key answers will be concentration skills and hand skills (strength and endurance). This is fascinating to me nevertheless.