What are these two wires used for?

I have a CPU fan with four wires. Red, white, black, and blue. I know that the black is the ground and the red is the 12V and this model of fan has PWM and fan tachometer but is it the blue or white?

save yourself headache and ignore colours on wires and look at the pinouts vs how it plugs in

http://www.allpinouts.org/index.php/Motherboard_%28CPU%29_4_Pin_Fan

another tip dont always trust silkscreens either, I was banging my head against the desk one night just to find out that chi-co got that backwards along with crazy colors on a ps/2 keyboard a few years back. I had to put the wires back and do a continuity test while writing stuff down on paper to figure out what brand they were smoking.

Put an oscilloscope on the wire and you'll figure it out soon enough