It's mostly speed and intended use, some quality (e.g. length of warranty).
The Black drives are the fastest, and I believe are the only consumer drives to have retained a five year warranty since the Thailand floods.
The Red drives are intended for always-on NAS use. I'm not sure if they're truly TLER like enterprise RAID drives, but they should be durable.
Blue drives are a typical consumer drive. Nothing special, but no serious flaws either.
The Green drives are low-power, and are the slowest of their lines. I have read of reliability issues if they are hammered by heavy usage, but they're fine for things like media storage and streaming.
These days I buy WD Black drives myself. They are the fastest of all of them, and there's...