if you used the lock alpha option, it would help you color without trespassing the lines

it works on every layer

for example, you have a blank page, then you make a background and add another layer to make lines, like... a circle or something simple, and when you click lock alpha, you will be able to color only what's in that layer, like, if you pass the pencil above it, it won't color the background, instead it's only going to color the circle!!