Here’s a screen shot tutorial of how I colored things in general for the commission I just posted. I start with some cell shading to get the lighting how I want and then blend it together with the b6 pencil smudge tool. I’ll go in with the round brush to soften things up too. And then just keep working things until it gets to a point that i like.
This isn’t the only process I use but its a fairly simple way to get started. I hope this helps!