Painting the entire tree in one color and adding another color in the center while the first remains wet creates a dark center and a glowing outer edge.
Before trying to capture the variety of shades and colors found in skin tones, practice using one color or a mixture of colors. This allows you to focus on building up a range of values without being distracted by color possibilities.
It is a first step book to begin watercolor. There are a lot of information about watercolor painting: brush types, beyond brushes (using salt, spray bottles, masking fluid, palette, paper towel, toothbrush, plastic wrap, and even a sponge for creating distinct impressions), color schemes and also building techniques step by step so that one can learn watercolor from scratch to painting landscape or human body.
When I first started to learn the watercolor painting, I began noticing the color transitions, delicate details that were there all the time but I haven't seen any of them. As I come to understand; to see things in detail, one should draw.