The browning is called toning, and it comes from oils from fingers that have touched the ball. The only thing you can do is to make sure it's handled with care from as soon as you get the ball...pre or post sig. Handle by the threads and or use gloves to handle the ball.
Value of the ball would depend on the sigs. I've seen people that wanted full price for toned Mantle balls, but I doubt they get the price since many good ones can be found without toning. Supply and demand is what really drives the price.
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