Gimp 2.6 is what I use. It's really simple to use.
The first thing I've learned was to never color on the same layer as the sketch lines. make a new layer to ink it, and another layer for the color. The color layer needs to be on the Multiply setting.

Already knew about the layer thing, but thank you! I shall try gimp!

Try to get 2.6. 2.8 sucks.

I already had 2.6 downloaded... though it seems it can't open .sai files, annoying since I've been working with layers in there and I don't want to lose them.

can't you save it as a .png and work with it on gimp?

Yeah I can, I was a derp and forgot I could save each layer as a separate file, so I can open them as different layers, hah!

Well, my original idea didn't work, I really don't want to lose my layers on sai... I also have no freaking clue how layers work on Gimp.

layering is important

Yup yup, I already knew that!

 cybercat has lots of good vids