Photoshop painting is divided to 2 processes. One is making selection, the other one is painting.
Making selection using marquee tool is not so difficult, just consider about where is highlight and shadow. Simple.
However, understanding several color mode in brush tool and layer is very important because each color mode shows different result on my coloring. For example, basic mode ‘normal’ just show the original color, but ‘screen mode’ makes the color lighter and ‘burn mode’ makes the color more darker. It is applied on every tools which is related to painting such as brush, burn and blur tool. When I colored it, I concerned about the reflected light in the shadow because it made a shadow more deeper and realistic.
I spend several hours for thsi work, and it was interesting work even my score was not so good..