I unlocked this appearance the other day 🙂 That took me some time. There are currently no way to track which Legion Bosses you need to unlock the Artifact Hidden Appearance tint (The one that requires you do all the Legion Bosses)
So I´m quite sure I ran some more than once, but granted, I´ve not been aiming directly to unlock it, since the colours are not something I often pick for transmog, if at all.
In fact, I had to hunt down a whole new set for it, since I have very little “beige” lying around!
A few tips I can share, that I discovered; You need both of the end bosses in Violet Hold, and it appears they only change weekly. No need for Nightbane in Karazhan. You can transmog over your appearance and it will still count.
As said, beige is not my favorite colour, and it was a challenge to find an outfit I liked to match too. It took me all the way to Silithus and to Temple of Ahn’Qiraj, for the Genesis set, but that was allright, because it´s been a while! And I never did manage to get my reputation with Brood of Nozdormu to exalted, but now I am on the road to that too 🙂
This tint is probably the only other version of the Night´s Vigilance Appearance that I will be able to obtain I think. I absolutely love the light purple version, but 10 wins of rated Battlegrounds seems quite far away from someone who rarely PvPed in Legion at all. I was a fan of PvP back in the good ol´ days, but not anymore.
I found a picture displaying all possible colours of it. Upper left is all Legion Bosses killed. Lower left is the original. Upper right is KilJaeden on HC. And Lower right is 10 rated Battlegrounds won.