Coloring skills evolution

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.

2) The child notices the image, and uses the crayon to color it. But he is not coloring, he's examine the image, points to the parts that interest him, and the crayon colors them. For example, the child first notes the face, doodles on it, when goes on to a next face/object that interests him on the picture (if any) or proceeds to the next picture. He can make as little as a few strokes.

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.

4) The child colors the image area by area, trying to stay in lines if possible, BUT forgets to change the crayon OR changes the color but just because he's tired of yellow and he wants blue now, not because the scene needs it. So the end result may be very similar to the stage (3) unless you can see the strokes or watch the process. The child will most often color only the main object of the scene or the one that interest him the most, leaving the rest of the page blank. (image on the left = 3.5 yo)

5) The child colors the image using different colors, thoughtfully assigning the color to a specific area. The colors are not always true to life (the clouds can be orange, and the trees purple). The child soon learns to use the yellow crayon to color all the areas that need to be yellow before putting it down and taking another one. He is likely to color the whole scene not just the main object. (image on the left = 4 yo)

6) The child may add his own details to fill up the blank page, such as a sky, clouds, sun, grass, etc. He makes the coloring book page look more like a free-hand artwork. (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.