1k kills for red skin...

[xuros]

does this mean 1k killing blows or just HK's?

Just HKs. Tag everything and you will be done in like a week. Shorter if you pvp more.

[Cosmic Janitor]

Heroic Xavius and Mythic Nythendra (I think, unlocked it ages ago) count as well.. I think I got 460 of the 1k kills during progression nights.

- - - Updated - - -

Not entirely sure how the dungeons one works, as it seems to be the only one tracking per weapon, while WQ and HK ones accumulate kills/finished WQs.

Dungeon works like the PvP one and the WQ one as well. The tracking is just done per dungeon, which means you need a macro that adds the individual hidden achievments together for the accurate count. Once you unlock any of the hidden recolors you can use them on all weapons on that character.

[Bathory]

Dunno if Ashran counts for kills but, you can get kills pretty damn quickly. I complete the repeatable "Slay them all - kill 200 players" a couple times when I'm in there.