What do the green and yellow wires in a BLDC fan do?
I don't know anything about that fan specifically but in general:


The tach line is some times open collector, that is it only pulls to ground.  Based on what you've said, it sounds like this is the case with the fan you have.  You need to use a pull up resistor on the output to get the expected square wave.
The green wire is most likely for PWM control.  Feed it a PWM signal at various duty cycles to control the speed of the fan.  Basically the fan has an internal transistor in the power path that you can switch using this input.  


See here for some background: http://www.maximintegrated.com/app-notes/index.mvp/id/1784