The fact that the red one (which should be positive) is turning brown, sounds like you are overheating the cable, causing the cable's sheathing to overheat/burn/melt, which could cause it to discolor.
This could be happening for a couple of reasons, one, you placed the positive wire (red) on the negative terminal and the negative wire (black) on the positive terminal, which would cause all sorts of over loading/shorting problems and possibly fry your entire electrical system.
The other reason this could be happening is you have WAY too much draw (overloading) on that battery (i.e. you have an amp, offroading lights, and a bunch of other stuff wired directly to it and constantly running).
I'm really hoping its the later of the two...because in all honesty if you can't tell the positive wire from the negative wire, you shouldn't be doing any work on your truck...
2003 Edge 2WD 3.0L
Air Box Mod | 3rd Tail Light Mod
Maglite Mod | Fog Light Mod
Headlight Mod & Clear Corners | Sport Trac Cluster