copy paste from wowhead artifact skin:
By evlen1291 (326 – 1·10) 2 days ago (Patch 7.0.3)
It seems that I had found a way to unlock hidden apearances without finding the secret thingy thap opens them up.
What do you need to do:
1) Unlock secret appearence for any of your artifact.
2) Unlock their new color (do 200 local quests for an example)
3) Use your forge in a spec with unlocked artifact
4) Press SHIFT+RMB on any other artifact in your bag
5) You can choose recolor of the hidden look.
P.S. You NEED to do it in a spec that has unlocked hidden apearence.
P.P.S. This really seems like a bug, but when I hover over recolor it says that "To unlock you need to do 200 locals using artifact with a hidden appearence"
This opens up new looks for all of your artifacts.
Here is a proof of it -
http://imgur.com/IzXjJoj I`m wielding third recolor of hidden Unholy`s artifact though it`s locked from me
Here you can see that initial recolor is still locked -
http://imgur.com/hGcJUF1