sjsquirrel made a tutorial about recoloring
scissorsandplanes's models with inkscape. Bruno's models are vector based (made in MS Office) and Inkscape is a free vector drawing program.
Note that you won't be able to use Inkscape to recolor a raster-based model. For that you'll need a raster drawing program (paint.net, paintshop pro, Photoshop, the Gimp etc.)