If it's rotting, there will be soft spots where it's beginning. If there aren't any, I still think it's just different colors of the wood. It's too uniform for me to believe it's rotting, though. I have a feeling that you will still see the different colors if you take it out and let it dry, but it won't be as noticeable. Most of the time, when wood gets wet, colors darken or brighten significantly from when it's dry.