Biggest difference is that WD Green is a little slower, low power, quite drive, while WD Red is good for network array storage.
Pegetsystem (link below) has a side by side comparison of the two drives - “Overall, the differences between WD Green and Red drives are very subtle. From a noise, power draw, and performance standpoint Green and Red drives are pretty much identical. Really the only major specification difference between the two drive lines is that Red drives support TLER (critical for RAID arrays), have a longer expected lifespan, and an extra year of warranty. Red drives also have a longer head parking delay than Green drives, which can either be a pro or a con depending on if you want to reduce power draw as much as possible or maximize performance and lifespan.”