I have done it by soaking them in brewing coffee with no ill affects. It takes a while but if you are patient it will give a mild aging to your grips.
I recently purchased bonded ivory grips for my M60. I know that over time the grips will age like natural ivory. Is there a way to speed up the aging process?
I do not see the point of trying to fake age a fake ivory set of grips. In short, you are trying to make a fake. Why? Spend more time and money on ammo, shooting and training and it will do more then a cosmetic change.