Fighting with Style: Hidden

This Feat of Strength is earnable after you unlocked your hidden artefact appearance.

To get the other 3 Coulour variations you have to do these 3 things, while having your hidden appearance selected:

200 WQ
100 Legion-Dungeons (Normal/Heroic/Mythic/Mythic+)
1000 Players in PvP killed (Arena/BG/Open World/Dalaran Underbelly)

P.S.: Im not sure if you can transmog your hidden artefact skin, and then get credit for the FoS

P.S.S: You are able to transmog your weapon, while still getting the count for the 3 Objectives.

You no longer have to wear the hidden appearance to get credit towards this.

This macro correctly calculates and displays the progress towards all 3 remaining color variants.

/run local a,x,g,b,y=0,11152,GetAchievementCriteriaInfo;for i=1,11,1 do _,_,_,y,b=g(x,i);a=a+y;end;print("Dungeons: "..a.."/"..b);_,_,_,a,b=g(x+1,1);print("WQs: "..a.."/"..b);_,_,_,a,b=g(x+2,1);print("Kills: "..a.."/"..b);

Edit: The dungeon tracking achievement did not get a 12th criterion for Return to Karazhan. I would therefore argue that it does not contribute to unlocking the 2nd color tint of your hidden appearance. Good news: the macro is still correct as of patch 7.1; Bad news: you'll have to complete 100 of the initial Legion dungeons.