Power steering fluid gone a funny colour

Ok, I've just checked my fluids and my power steering fluid has gone a nasty brown/black colour. Turned fairly recently, since I check the fluids regularly.

Any ideas??

I had to run a couple of changes through mine before it stayed clear. I was switching from whatever the dealer puts in to Castrol Syntech. I wonder if the different fluids reacted somehow? It's stayed clear for many months now.