Pretty simple trick, you need the secret appearance unlocked of either assassination or subtly to do this trick, also another color of the hidden artifact.
- Unlock secret appearance for any of your artifact.
- Unlock their new color (do 200 local quests for an example)
- Use your forge in a spec with unlocked artifact
- Press SHIFT+RMB on any other artifact in your bag
- You can choose recolor of the hidden look.