The bottom cord is black and, according to the picture of the power supply, it is the ground (or Earth) wire, marked as GND in the power supply picture. The top one looks purple in your picture, and according to the picture of the power supply, that line is +5VSB. VSB on a power supply is voltage standby, so even when the power supply is in a low power mode (like "sleep" or "hibernate"), that line still has 5V running through it.
EDIT: The primary use of the +5VSB line is to retain memory data during standby/sleep/hibernate mode. It can also be used to power the motherboard circuitry that does a "soft power on" (when you press the power button). If it isn't plugged in, then the memory is being retained during sleep mode via a cache on the hard drive, and the soft power on is using full voltage, which doesn't hurt anything (as far as I know).