Inkscape surely is used by a lot of repainters!! It's a great open-source, free application which is very dynamic, lots of tools and features that come in handy. I've seen many new, young repainters pick up the software and quickly learn how to reproduce great liveries.
There's a fantastic guide demonstrating how to repaint Bruno Vanhecke's fabulous small scale models, the same techniques can be used for larger models as well.
Post:
http://www.papermodelers.com/forum/408833-post1.html