OK, here's what should be done.
First, you have to locate the HUD's .tga files. In case of HUD-equipped add-on aircraft, usually they can be found in the AircraftName\Cockpit folder. However, in case of stock aircraft, you have to extract them with the help of the cat extractor. (you can find it here: Cat Extractor )
Launch the extractor, go to the Objects folder, then open ObjectData.cat. The avionics.ini (also located in ObjectData.cat) of the aircraft in question might help to find out what files you need to extract. You will find the extracted files in the Objects folder.
When you've found the .tga files, you have to repaint them. You need a program that can handle .tga files (such as PhotoShop or PaintShop). Then paint the files to your liking. The repainted files should go into the AircraftName\Cockpit folder.
Hope this helps.