I think you are talking about Paint tool sai with water color brush. I may be wrong, but alot of artists, including me, uses a mix between regular brush and water color brush.
Well in PS, you can add a texture in your background, to make it look like you're painting on a rough paper, and then use an opacity brush. A very soft brush (with wet edges) should be perfect! I don't have Gimp but I know it's similar to PS. Maybe you'll find it!
I don't know about GIMP, but I'm sure you can download some GIMP and / or ABR brushes that give those kinds of effects. Just search deviantart, "Watercolour brushes for GIMP" or the like.
My colors right now look super flat and boring
I use Gimp and have a tablet btw!