As galeener mentioned, the wd green drives are energy efficient lower power drives (like a green car). They're slower, 5400rpm drives and not really suitable as a system drive. If you need inexpensive lower power storage for bulk file storage then it's an option, but typically for things like music collections, family photos you don't plan to access that frequently and that sort of thing.
I wouldn't recommend gaming on them. Wd blue drives are more suited for that as inexpensive mainstream 7200rpm hdd's. Pricing may depend on your location though in u.s. pricing the green's aren't all that cheap. The wd blue are a few dollars cheaper and the re4's can even be had for similar price, around $52 which is a bargain. They used to be around $120.
The re4's if you can get them cheap are solid drives, enterprise (similar to wd black) with raid in mind but work well as single drives also. The wd blue and green have 2yr warranties, the re4 have a 5yr warranty. They also undergo burn in testing, have twice the load/unload cycles as even the similar wd black's and are designed more for enterprise systems with 24/7 operation. I believe the wd black and re4's offer 1.2m hours mtbf vs a lower amount for the blue and green (around 750k hours mtbf if I'm not mistaken).
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