Correction: The WD Green drives rebranded as Blue, with a trailing Z on the model number.
http://techreport.com/news/29251/western-digital-paints-its-green-hard-drives-blue-in-rebranding

Reason for wanting a Green/Blue drive: single drive, not part of RAID, so the drive should try harder to recover from errors.

Reason for wanting a Red drive: part of a RAID array, so the drive should give up sooner on errors, allowing the RAID to handle recovery.