I found this on the internet (not sure how useful it is though)
https://www.poweradmin.com/blog/dell-poweredge-2950-server-quieting-the-loud-fans/
Red +12VDC
Black 0 Ground
Yellow Tachometer out RPM
Blue PWM signal in
from this site
https://www.dell.com/community/PowerEdge-Hardware-General/PowerEdge-1900-fan-noise-solution/td-p/3024592