Either sun bleached white antler or bleach it yourself with chemical chlorine bleach... though oxy-clean may also do the trick. Keep in mind that if you are turning down into the "pith" of the antler, where the marrow shows, nothing you do will make that area look a perfect white, so keep to the outer sections of the antler tines where the marrow lines in the center are thinner.
Last, but not least, in order to try to make sure you dont have marrow showing up halfway down the blank, you'll want to do the trick where you soak the antler section in a solution of hot water with vinegar in it (not sure how strong you need to make it), and then gently clamp the antler section in a vice to straighten it out and let it dry that way. Take a PVC tube about the size of your antler section and split it down the middle, and mount it on your vice jaws so that you can clamp even a compound twist.
Also keep in mind that the vinegar solution will weaken the calcium of the antler, so you WILL want to coat it with CA for your finish when you're done.
If you want to avoid all the mess and trouble, then I suggest you opt for plain white or opalescent white acrylic or other type of resin that is a guaranteed white color.