WHile there are many tutorials on repainting, the basic programs that you need are:

A. A texture converter - such as Martin Wright's DXTBMP or Microsoft's Imagetool.exe.

You can donwload DXTBMP from this site (*check the Simviation Design Utilities -link in my signature*)

Imagetool.exe comes with the scenery SDK.


B. A paint program.  While MS Paint will do, it is highly recommended that you use a decent paint program - Gimp!, pixia are freeware.  Many people use programs like Corel Paint/Draw, JASC's  Pain Shop Pro, Adobe Photoshop.


C.  Commercial Software:
Abacus' FS Repaint!  is the only software -commercial or freeware - I know of that will allow you to view an aircraft model, make a variant and copy the textures to the new folders, and allow you to repaint the textures and view your changes on-screen as you make them.  You can paint with a built in editor, or in conjunction with Corel Paint or Paint Shop Pro (up to 7).