I have been restoring hard rubber for about 25 years now. There are a lot of things that can cause oxidation. Basically the sulfur migrates to the surface and when it interacts with Oxygen, Sunlight, Moisture, etc. This will turn the surface brown.

You should never use water on hard rubber. It is very bad for it. If you have a brand new gun or one that was well stored and the grips are black you want to treat it with anti-oxidants. Mineral oil is very good. So is carnauba wax.

I actually make all my own products since there is nothing made specific for hard rubber. Recently I also made a deoxidizer that will remove the oxidation from the surface as well without harming the rubber.