Oddly... the fluid in the reservoir was filthy (as per the picture), but the stuff coming out of the calipers was fairly clean, just slightly darker than fresh OEM and a bit cloudy, looked a bit like olive oil.
I think it was changed 2 years ago when I had the brembos fitted. I did cook the fluid last year though.
I pulled 250ml out of each side of each brembo ( so 500ml per caliper) and about 300ml out of each rear. Probably far to much but at least I know its all fresh now.