Here's a tip:
before you start to draw, always make sure to create a second layer to do lineart,
then create a third layer
[after you're done with the lineart],
place it below the second layer
[or move the third layer below the second layer if that makes sense],
go to the fill tool,
set tolerance and grow at any number you need it to be,
[I always set tolerance at "9" and grow at "2",
that way the space I'm filling in fills/colors the area in nicely]
then tap/click the area you need to fill/color in.
hope this helps
[sorry for deleting a comment, I forgot to mention something when typing this]