What I typically do for recoloring CSPs is putting the original CSP in Photoshop, opening the costume in brawlbox and display it, alt+print screen, paste screen in Photoshop, sample the colors from the screen and copy values into notepad (usually 6 numbers/letters), select image>adjustments>replace color, sample original color, set second color using value in notepad, make adjustments, and voila! Basically, select region and make color adjustments like hue, saturation and brightness in whatever you got for image editing. You can probably do the above steps in Gimp, but with similar tools that Gimp has.