In a compaq power supply the ground wires will probably still be black (I hope).
There will be two wires in a compaq power supply with a positive voltage in respect to those ground wires when it is off, the standby 5v supply for soft power functions on the motherboard and the PS_ON wire which you want to ground. So basically, find these two wires, connect one of them to ground. If it doesn't work, connect the other one to ground. Since all ATX power supplies should have short circuit protection there isn't any realy danger in shorting the standby line, though you should flip the switch off and back on in the back of the power supply if the first wire didnt' work.