Are you sure it is Browning from getting too hot? I use paper towels as substrate and occasionally when a snake urinates its a amber/brown color that looks exactly like your pics after it dries. I'm not doubting you or anything, just thought maybe it was new to u.
I agree that its weird that yur heatpad can get sooo hot while the basking stays lower like that. How many layers of paper towel are you using?
Sent from my SAMSUNG-SGH-I997 using Tapatalk 2