I actually REALLY like the coloring in this picture.
While it is simple and there isn't much to the tone in the shading or multiple light sources, it's not even needed.
I feel like it's something with the colors you chose to work with, and the way you colored certain parts so stylishly. For example, the way her lips are colored make them POP out from the simple skin tone, and the hair captures the waves well, without being overly detailed. Normally I don't like such simple pieces.. but the simplicity is what makes this piece beautiful! Good work on this!