Jack Jack Frost Frost

Cool! Jack Frost!

I could definitely give you some advice on this.

I would say, first thing is you should try to stray away from simply using local colors for the skin. Skin has a lot of variety! I would experiment with different saturations (everything is kind of in the grey scale, which isn't bad, but you can strategically add more saturated color where you want things to pop). Also, you can push the shadows to be a bit cooler since the overall feel of the skin is quite warm.

Another thing, definitely think about cast/ soft shadow plays on each other. Especially since you're using sai, sai has beautiful brush stroke, so be confident with your cast shadows, make em hard-edged and full opacity. When a hard light/ cast shadow go up against soft, rounded, shaded forum and blend into one another, it makes it so sweet haha.

But yeah, overall, just think about color temperature and shadow play/ stroke confidence. Just keep on doing what you're doing!

And cool job by the way! Hope this helps.