normal on a duc. i wouldn't worry too much. millions of theories on this. i go with the heat thoery myself.
ATE TYP200 (DOT 4, but with DOT 5 boiling point) seems to last longer in my bike before it turns black.
I just change mine whenever it gets too dark. It's much easier than doing the brakes. I put a drain hose on the slave bleed and just pump new fluid through until it has all been replaced. No need to keep pumping and nipping like brakes. Takes five minutes.