Killing a wither skeleton while wearing a wither skeleton head does not give you the Camouflage achievement. I was on a real which had achievements and I put on a wither skeleton head. I then killed about five wither skeletons while still wearing the head. I did not receive the achievement. I assume that this is the same on normal worlds, but I don't have a world that I can do this on, Just the realm.
To reproduce:
go into a world/realm with achievements
put on a wither skeleton head
kill a wither skeleton while still wearing the head
observe if you get the achievement “camouflage” or not
I just got the achievement now after over an hour later. I did end up getting the achievement, but very late. I did not compleat the achievement over again.