I have used yellow with a red stripe to indicate pos, and regular yellow for neg. I have also used White for everything, normally it just boils down to how much of what wire i found in a dumpster, and how much do i need for said project.
My homemade PSU, would drive anyone mad, cause i have colors cris crossing each other. if i was not the one who made it, i would not make head or tails of what was going on with it..
but for alot of low volts i have seen this. RED pos, Black Neg, Yellow or Green as Ground.. but for my PSU, the power brick, the 12v is yellow, and the 5v is red, black is ground, and blue is what i have to short red to, to get it to stay on, or use it as my switch.