I have tried this today, the antler had no colour left in it really, almost completely white.
I mixed up some potassium permanganate, about a teaspoon for a cup of water.
I have heard you can use tea and WD40 but i decided to try this.
Don't be alarmed if you try it, the stuff goes on purple but after a few minutes goes brown. Do not get this stuff on the skull, you won't get it off, so i used some masking tape.
I tried on 2 skulls, i prefer the set on the left as i only gave them 2 coats, having said that i did shoot a buck earlier this year that had a dark brown set of antlers like those on the right.