Used to be green and none green drive to choose, but now you got blue and black too, both running at 7200rpm.
Blue is only up to 1TB, slightly faster in read and write, but with 2 years warranty.
Black is above 1TB (not sure there is a 1TB black) with 5 years warranty.
If you are going for 3TB, looks like the black WD drive will be the choice.
Edit : my info was out dated, as there are Blue drives above 1TB.
Edit 2 : however Blue above 1TB spins at 5900 rpm!! So watch out.