Typically such wires are green, but not always.
That would be my best guess anyway. Probably to help cut down on the amount of interference from the power wires, so that the RPM data is more reliable.
I would assume, though I can't say for sure, that any 4-pin fan you buy would work simply by plugging it in. Just be sure to have your finger on the off button if you plug it in and it doesn't spin up. You probably have 5 seconds or less before your CPU fries. Don't rely on the power button on the front, flip the switch on the power supply. If that switch isn't present, hook the system up to a power strip that has one, or pull the power cable out. You want to be able to cut the power as quickly as possible.
You can also try emailing Dell tech support, see if they can't tell you what that wire does. You can also try more hardware oriented sites, like the forums on Tom's Hardware. I have only a slightly better than average knowledge of the low level electronics stuff. There are bound to be people there with knowledge that dwarfs mine, and probably a couple of people who think a great way to spend a Friday night is to poke electronics with a multimeter to see what is connected to what.
Do know, however, that you will void your system's warranty by doing what you're doing. So I wouldn't undertake this unless the system was already out of warranty, and also keep in mind that you may completely fry your system by doing this.
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