12VDC Computer Fan - Control speed

Hello,
Currently i have a fan blower in my computer that has four wires (Black, Blue, White, Yellow). I want to add Fan speed circuit by controlling the voltage to the fan. Is this ok? The computer monitors the fan rpm using one of the other wires i believe, because i have screen that give me the read out. The blower runs at a very high rpm and is therefore very loud. I understand that @ higher rpm's it helps to cool more, but i wanted to lower it a bit to help with the noise.

Hi there,

I lowered the voltage to one of my 12v fans and it worked great.
I used a LM317 with pot to control the voltage.