Lineart and coloring are done in two separate layers. I do all of his coloring/shading in one layer so it’s easier to blend skin colors. When I’m finished, I combine the two layers.
(When you’re coloring you can set the layer to Preserve Opacity. It keeps everything between the lines for you!)
In almost all of his pictures, I make a new layer set to ‘Clipping Group’ above him and Overlay a rich color to enhance his palette, but this time I set it to Screen instead. That’s all folks! :)