I see you're requesting critique on this so I thought I'd comment on some things I think would help you!~

• The first thing I noticed is a lack of a definite light source. There's value, but now shadow. Another thing that sticks out is the fact that you only used the lighter and darker versions of a color to shade. Never ever do this, it makes a picture look really flat and boring and our brains like seeing lots of different colors to excite it.

Try experimenting with colors, make the shadows a cooler color and the highlights a warmer color. Another fun thing to play with is making the layer you're doing the shadows on set to "multiply" while using a deep saturated color. It has a really nice effect - try it and you'll see how it works~