I assume you're lookin at Western Digital internal storage drives, correct? if so then the Blue series is aimed at individuals who want a balance of speed and performance while the Black series is geared towards performance. This is why the RPM's of the Black drives are 7200 and above while those found within the Blue range are 7200 max. Green drives are meant for pure storage, with less power draw and less heat output which is why their rpm is found to be at 5400RPM's max. Red's are for NAS(Network Attached Storage) and their Purple range is for surveillance gear.
Now, what are you looking for?