The reputation faction related to the Kul Tiran Allied Race is the Proudmoore Admiralty though, I guess that’s why Blizzard didn’t take the Waycrest and Stormsong colours into consideration.
The thing is, Blizzard gave alternate colours to the Mag’har because the Blackrock and Frostwolf clans are well-known and iconic. The Waycrests and Stormsongs? Not at all, they were introduced in this expansion. The average player associates Kul Tiras with the colour Green, I guess that this was Blizzard’s though process.
If you look at Lightforged Draenei, they were supposed to have a purple variation of their Heritage Armor, but it wouldn’t make sense. You don’t associate the LIGHTforged Draenei with Purple.