What software do you work with? I primarily use Clip Studio and have been learning more about coloring techniques recently. Clip does seem to have a pretty smart & adjustable fill tool that can deal with some line gaps. In the couple practice pieces I’ve done so far, it made flatting so much easier for me than it used to be!
I'm using paint.net, a freeware paint program. I actually installed clip studio and was playing with the trial version a couple weeks ago. It looks really cool (especially being able to move around wire-frame models on their own layer). Bu I don't have the patience to learn how to use it properly. One thing that is conspicuously absent from paint.net (that I like using in other programs) is there is no Blending tool. Doesn't affect my comics that much, but if I'm working on a more serious painting it's something I would like to have available...