Well, that is easy. You take a large brush, some different coloured paint and proceed to paint .
Seriously, depending of which version of TRS/TS you have, open its installed folder and look for the texture map(s) of what you want to modify.
With a good paint program, repaint the areas on it you want to repaint/modify and save the texture map(s) in the same format as you found it. If this is a .TGA format, make sure you save this "uncompressed" or you will only see a white repaint in game.
Try using a program called GIMP. Usually you need to look for the main image file which are usually in TGA format or BMP format. They are usually called 'main'. after you save them you need to open and save them again with a program called infranview. This allows for the image to be uncompressed.