This is an unused look. This appearance has never been available on a player mount.
Notes:This colour appeared on the 5.2 PTR, but was subsequently removed from the database for some reason. The replacement (in-game) purple is quite different.
There are four possible harness colours for pterodactyls, and they aren't matched to a specific mount colour. We don't know which Blizzard would choose if they do make a mount in this colour, but all harness options may be browsed in our picture viewer.