Hiya Graeme,
Well, you need the original (or another repaint) skin file which you open and edit, using something like paint, or psp, or PS, or any other program like that.
How to go about it if you need to extract this from an addon, convert stuff from one to the other and back, and such, is maybe a good question to ask on a Flightsim forum where they have an addon creators section, perhaps? I reckon the help you can expect on one of these might be more fine tuned to the specific sim...
Regards,
Fred