Color is not the only thing that can vary in a biped. Sometimes jackals have mohawks and marines have hats. There have been multiplayer 'perms', as they're called, in stuff like backpack weapons and different helmets.
This is done my compiling two different 3D models into a single gbxmodel tag. You can certainly only change the textures if you want; just have one model have one texture, and the second model be almost the same, but with different textures applied.
Color change in MP is done with function stuff and things called multipurpose maps... basically, there's a bitmap that shows where you want the player color to show.
You can script which perm you want to use with a script command. I made these things, in like, July: http://forum.halomaps.org/index.cfm%3Fpage=topic&topicID=32883
Edit: Does want Snickers.
Edited by SlappyThePirate on Dec 3, 2010 at 11:09 PM