Google, potassium pergament, it is used as an oxidizer in cleaning up wells and ponds. It will turn the bone or antler brown depending on how its used and to what degree. A word of caution it turns everything brown skin, hands, clothes and it will not wash out, if you get it on you its wash and wear. So it is best to use gloves. It mixes with water to make a liquid and looks purple, however as it dries it goes to a rusty brown. A quick buff with a rag knocks off any of the rusty looking residue. Then it is up to you by the amount of coats you apply and how you buff out the color you achive!
It is getting harder to find because no one wants to ship it and can only be shipped by ground after the accident on the plane in the Everglades, (no air shipment for oxidizers)!
Here is a link to a company that sells it. Go to chemicals and then to the P's and you will find it.
http://http://secure.sciencecompany.com/Chemicals-C665.aspx
However you can buy potassium pergament off of EBay cheaper, also in the powder form.
I have had limited success with oil based paints like artists use, also some with oil based dyes, as well as letting it sit submerged in coffee or tea for an extended period. The problem with the liquid is that the antler needs to thoroughly dry again.
Below are pictures of an antler that was a pure white shed when I started. Although I loved the character of the
popcorn on the antler, it had way too much
popcorn on it to be comfortable as a handle. I started with a base coat of black oil paint rubbed it on and wiped it down to a look I liked. Then I buffed with 220 and a greenie pad to remove some of the
texture/popcorn. Those areas became white; it was then treated with potassium pergament. The brown color is the result of that treatment. I them buffed lightly with a greenie pad again and sealed with CA (superglue). When I put the handle to the knife I will polish again and bring more of the white back into the handle. That will give me a contrast of the black, brown and white against one another and then seal again with CA. Its kind of like painting a picture the background is done first.
Click thumbnail for larger view:
Hope this info helps!