Your coloring is excellent! I very much enjoy your simple shading as it is very lovely and not too heavy. I love the dragon's expression as well as his color scheme which while bright, is easy to look at. (I understand it is a comission but some days its hard to get a color right even with a reference! I applaud you!)
I wanted to suggest in future drawings that you may want to try making the area where the wing joins the body a little thinner. It takes up a lot of the back and if you make it smaller you can show some of the dragon's body a little better and make his neck look a tad longer.