Personally, I'd be tempted to use AutoCAD, but that's likely because I'm way more comfortable using that than the GIMP.
I should really get more fluent with image editing software, but there's only so much free time, and I used AutoCAD professionally for about 6 years, give or take.
If you want to go the AutoCAD route, bug the crap out of me, I could knock together a template for a Filco/Leopold/Noppoo-esque generic keyboard fairly rapidly, and I think that if I layered it right, you could change the color of the keys pretty easily, and also apply global font changes etc. by assigning a few dimension styles to the various parts of the board (e.g. one dim style for the alphas, one for the numbers across the top, one for the numpad, etc.) so if you changed the font for that dim style it would take effect over that whole group of keys.