Gigantic file ahead! I made a new revision of the old tutorial because I hated it. Though it was funnier. xD Please excuse any grammar or spelling mistakes for I'm not fixing them, and I hope this helps you. <3
This is really wonderful. I like how you modify the color of the original outline. I'm left with a couple of questions, though: what's the purpose of the second layer in the second step? Is that just so you don't accidentally mess up your outline? Also, what do you mean by preserving opacity in the second step?
Yes, the second later is so I can freely color without worrying about mession up the lines. c: And by preserving the opacity, you can't color outside what's already on the layer when you preserved it, so it comes really handy for step 4. If you mean how to preserve the opacity, it would depend on your art program. c:
Click the grey/white checker box by your colors c: It's transparency, and unless you have 'Preserve Opacity' checked on your layer it'll draw like an eraser version of whatever brush you use. That's what I do c: I haven't found a shortcut for it yet, so your way might be quicker, but I find it's easier because I can use it for my regular style too. I never use the normal eraser