WD says this about the green...
WD Caviar® Green™
Cool, quiet, eco-friendly.
Select green for doing more with less energy consumption.
Reduced power consumption - WD has reduced power consumption by up to 40 percent compared to standard desktop drives with the combination of WD's IntelliSeek, NoTouch, and IntelliPower technologies.
Helps enable eco-friendly PCs - WD Caviar Green drives yield an average drive power savings of 4-5 watts over standard desktop drives making it possible for our energy-conscious customers to build systems with higher capacities and the right balance of system performance, ensured reliability, and energy conservation.
Cool and quiet - WD GreenPower Technology™ yields lower operating temperatures for increased reliability and low acoustics for ultra-quiet PCs and external drives.