hm...maybe the lines are not closing everywhere? or...be sure to check the box 'Target: working layer' and don't accidently go on a different layer...oh and maybe it helps to set the 'transparent difference' (transp diff.) lower, like...25 or so. I hope that helps ._.
This is a good idea and will be helpful. I have been struggling with colouring in Photoshop, which is kind of new to me. Even if it's "your way," it is ten times better than my way. Thanks for posting.