Not using a 360 PSU at all. On my PSU case, it clearly states that the purple wire is "PS ON".
Edit: I just checked the PSU in my Internet PC, and it states that the Green one is PS on. But I was not wrong, the PSU in my cooling box says that the purple is PS on.
My cheap chinese PSU says that the grey is PS on. But I just looked it up, and green and grey are standard for PS ON. I guess it's mostly brand specific. So make sure you check you PSU case for the colour code.