After outlining the tree, I added colors on the outline, which will add a sparkle effect to the image. More colors next to each other, the better the effect. I also painted the dark shadows making sure the colors are not bright and doesn't have the "out of the tube" look. To dull a color, the easiest way is to add it's opposite on the color wheel. If the green is too bright, then I would add a little "red" to tone it down.

