I have several WD hard drives.
A green, a blue, and a black one.
The blue one I use as a media drive (music, images, and videos). Its read / write speeds are decent and it doesn't run too loud.
The black one is slightly louder than the blue (not that loud though), but it's read / write speeds are slightly faster. I use the black drive for gaming (storing game library and gaming clients).
The green drive has lower read / write speeds. I'd class the green drive as a basic range hard drive. I use the green drive to store my OS and some programs on.
To Summarise:
Black > Blue > Green
The are also red and purple drives. I believe these are designed for enterprise storage and surveillance storage.