Step 8:

Start working the background. For large areas, I will sometimes use selection tools and flood the areas with color (alt/opt + delete will fill with foreground color).

A note on the selection tool:

You have to be on the "lines" layer to select areas of the drawing. If you use the quick select (my selection tool of choice) on a blank layer, it won't see any borders and will start selecting random things. But make sure you jump back to your coloring layer before you fill the area; otherwise, you'll be painting on the (multiplied) lines layer.