Coolant changed colour

I just noticed that coolant changed its colour from blue-green to kind of yellow-rusty...last time I looked was the other week and I would have noticed if it was any different from normal...
The coolant is Motul Inugel http://www.opieoils.co.uk/p-904-mot...ng-liquid-anti-corrosion-and-anti-freeze.aspx and it was changed ~20 month ago (~20k km) so still should have a bit to go (was planning to change it at "autumn service"...
Level wise it is perfect...no traces of oil contamination or coolant in oil (all levels as they should be)
It's clear- isn't murky etc.
Temperature wise it's always been perfect- 85-90C while moving (have the OBDII gadget that reads the sensor and displays on garmin)...cooling system seems to work perfect- give it a little stick and the temp will drop slightly...car never has overheated or even hit 100C...
Any ideas why?
Is there a pH indicator that changes colour when the coolant is on the way out?
I assume it is a good idea to change it but is there anything else I should do/look for?

Usually turns that rusty brown after it has mixed with a bit of oil afaik.. Could be a head gasket on the way

I hope not...no visual signs of oil in the cooling system (checked the top of the rad) and the other way around...

rusty insides? here this happening when you use tap water instead of distilled water/coolant in the system

It's a pre-mixed motul...it isn't sludgy just as if the green dye faded away...

Probably age related. Theres little testers you can buy in Motor Factors

that test the quality and strenght of the antifreeze/coolant in your car.

(there like eye dropers things, suck up a few ml of coolant and indicator tells you)

Only a couple of euro too.

cooling system serviced and thoroughly checked with no faults found...sorted...