you may be better off posting this in the repainting forum, but ill help if i can anyways.

Im guessing you are using fs9 and so you will first need a program like DXTBmp to convert the files used into regular bitmap images.
You can now edit these bitmap images in whatever paint program you use. I would suggest Paintshop Pro but there is some good freeware products like Pixia. (see the repainting forum for links and the like)
In this program you can paint it as you like and then you need to convert it back, using DXTBmp, to a DXT3 file.
This should change the paint scheme for the next time you go on fs.
There are some Excellent tutorials around such as:
http://www.gjsmith.com/Tutorial/Tutorial.htm

hope ive helped

ant