You could try giving it a clean with some diluted multi surface cleaner to remove any oils, etc. I have used Eiszett Gummipflege (which is designed for car seals)with some success to restore rubber grips, though they where not in as bad a condition as you describe. There are a number of rubber/plastic colour restorer products designed for cars on the market which might work as well. Use at one risk, etc of course! (a well used trick/hack in the car industry is peanut butter for restoring colour btw)