You mean like my pink Dark Flow and purple Psycho Wand?

You can get hooked up with Photoshop, the .dds extension plugin, and a texture manager and extract the .dds files, modify them in Photoshop, and re-import them to the texture manager. I could have sworn I had it working in GIMP before - the freeware Photoshop (which I actually know how to use better than PS) - you may need to hunt around for plug-ins. Anyways, PS's import interface IS much better than the GIMP one regardless...