4 guage is used in most decent amplifier installs and is available in colours, usually blue, sometimes red.
Below 4 guage is the welding cable that is in black only (to my knowledge). I used 0 guage in my build to run to the rear of the cab then distro blocks split it up into 4 guage runs from there to amps, etc. You can use red electrical tape instead of heat shrink to mark the hot wires as well.
http://img101.imageshack.us/img101/442/101510.jpg

