Color change on Dial of Dateadjust -

I'm a newbie here, but I bought a Date Adjust (Silver) approximately 11 years ago and very seldom wear it. But recently the White face dial changed color to Copper. It's kind of odd--Where the hands were stopped it looks like if you were wearing sun glasses and got a sunburn.

Any suggestion or reason behind this? I'm assume that the warranty is already gone and what is the avg. cost to replace the dial then?

Thanks!
KC