Last year I wrote about Western Digital color-coding their hard disk drives to make it easier to find the optimal drive for an application. We saw that blue is for everyday use (about 40 hours per week in a PC), black for high performance, green for low power, and red for NAS (continuous operation, low power, vibration tolerance, error correction, streaming).
Now Western Digital has a new line color-coded purple. Purple is similar to red: it is aimed at surveillance video and meant to be always on, be deployed in bunches (vibration control), and used for streaming (different cache optimization). Although you could use red disks for video surveillance, the purple disks have an AllFrame firmware technology that reduces video frame loss.
Seagate's corresponding Surveillance HDD line has a turquoise label.