WD Blue HDDs are mainly used for general purpose storage, they are great all-rounders, and an average user can’t go wrong with Blue.
WD Green is basically same as Blue, but it lacks a bit of performance. However, it is the most eco-friendly HDD, and it’s quite cheap.
WD Blacks are for pure performance including gaming, media, and content creation. Also one of the most expensive HDDs.
WD Red is a NAS drive. It’s optimized for NAS systems.
WD Purple is for surveillance. Advised for 24/7 operation.
WD Gold, probably the most premium of all. Made for a multitude of Datacenter-specific applications.