I don't use black for shading. It's just the outline for contrast. I use a lighter shade for outlines near highlights but I really like my style for outlines. Thanks for the advice though.

I'm no expert but i'm not sure i like how you've used the black outline for shading, surely if you're going to shade the edges use a darker and lighter colour from the palette to do so?

Thank you for pointing that out :)

There are a lot of unnecessary shades here, and some more shadows to give the feeling of depth would be nice.

 There's a white pixel in the flame