In my work I create and save selection areas to paint inside. Take a look here at how you can do this:
http://www.simmerspaintshop.com/foru...nels-gimp.html
And also, if you don't have my GIMP brush set already, consider getting it. Here's the download link:
http://www.simmerspaintshop.com/forums/file135.html
So... once I've got my selection area layed out, and it conforms to the shape of the area you wish to paint (thus preventing overspill) then you can use the flood fill inside that selection to get started, then I use an appropriate softness on a brush to add any camo or other markings over the base color (I usually do this on a new layer above the base color, but still within the same selection area). This lets me adjust each color area separately later on, if needed.