Here's the tiers, WD goes: GREEN->BLUE->BLACK->RED
Green are considered Eco friendly, low speed drives. These are generally good for offline storage, USB enclosures, or backup devices/etc.
Blue is your "standard" desktop hard drive, general day to day use.
Both blue and green have a 2 year warranty.
Black is WD'd premier Desktop HDD, higher speed, designed for basic desktop use with a longer warranty. It is not Raid certified.
WD Red are basically WD Black drives with Raid aware data circuitry designed for a larger, faster buffer to prevent loss.
WD Red and Black drives both receive up to a five year warranty with credits: