The Green drives use what WD calls "IntelliPower", which essentially means that the drive will use whatever speed it finds "necessary". According to tests, the max speed is 5400 RPM, but it can "boost", or ramp up to 5900 RPM if the drive feels the need.
According to WD themselves IntelliPower is:
A fine-tuned balance of spin speed, transfer rate and caching algorithms designed to deliver both significant power savings and solid performance.
Basically, the drive will throttle itself when needed and run around 5400 RPM "normally" and in day-to-day use and only boost say at system startup or when the drive is used really intensely; like when installing large late, i.e. a game. It will still always be slower than a Black.
Whereas the Blacks run at 7200 RPM.