Even though it seems impossible, Dorian believes that his portrait will age while he remains young. He thinks that the painting will serve as his conscience, reminding him of what he has done right or wrong. He feel sorry for the painting then, and thinks, "Its gold would wither into gray. Its red and white roses would die. For every sin that he committed, a stain would fleck and wreck its fairness."
What does "wither" mean?