Control speed of Nidec fan?
The blue wire is a PWM control signal which can be used to adjust the fan's speed. The yellow wire is a PWM speed sense that can be used to read the speed out of the CPU.

To control the fan's speed, you need to give it PWM: a square wave adjusted to the speed you desire. This is doable with a microcontroller's PWM peripheral.

I'll direct your attention to this article:


4-pin Fan Pinouts