I'm not extremely familiar with GIMP but if you have the capability to mask layers I would recolor the work on two separate layers and mask out the rest of the image so the only colors that change are the blue and red cloths.
I actually had better results with hue/saturation, but the same technique still applies.
Like this: http://i.imgur.com/X7QdHvR.png
and voila! http://i.imgur.com/WyBx7Wm.png
(sorry I can't post photos yet, but the first one should give you an idea of what I did)
This post was edited by TheAxiom on Aug 29 2016 09:21pm