Had my water reservoir replaced today and re anti freezed on mondeo and just noticed after a day of care free motoring that my coolant is now brown.

 

Is this anything to worry about or is it just rust from the change of water ?

Try flushing the cooling system a few times and it should clear up.

I wouldn't want to be driving around with all that rust build up in the coolant. It could cause something to block up and cause the car to overheat.

That's your engine turning to liquid, as luke says- give it a good flush with a hose until the water runs clean(er).

Some do recommend some sort of cleaner such as vinegar, oxalic acid or dish soap to purge the rust particles but would need running for a few days which leaves the engine to risk of overheating (maybe not too much of a major risk at this time of year).

The problem with rust in the cooling channels of the engine block is that they clog the galleries and reduce cooling efficiency anyway.

TBH, most anti-freeze mixes have a rust inhibitor mixed in anyway, maybe the system has been topped up with water too many times but not quite enough coolant.