Open up HMT and open a map from your halo folder, or disc (I think, I don't mod my xbox) go to the bitmaps tag thingy in the file, and go down to the tag vehicles/warthog/warthog and click the black button on the screen 'save texture' and open it up in photoshop (the saved texture). You will need the .dds plug in from this site to open the texture in photoshop. Edit the texture in photoshop anyway you like, and save as whatever DXT number that was listed in a white box above the preview of the texture in HMT. Inject back into that tag file vehicles/warthog/bitmaps/warthog and you're done. the tag in HMT should look like this:
BTW you are going to have to do somethig special to mod your xbox, and I don't know how to do that, but this is how you would mod a skin for the warthog (or any skin for that matter).