Potassium permanganate can be used but be VERY careful as it will dye everything it touches. Its a purple color when wet but dries brownish. You thin it down with water (its a powder) and wash it on and wipe it off the antler to build layers of color. Just remember this stuff stains your antlers and there is no getting it off.
You can also wash it with oils paints, described like above. A little safer as you can wash it off normally before it dries if you dont like the color.
If they are not bleached to bad a furniture polish might give them some color.
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