Sounds a bit dubious and from a private seller who knows very little about about cars. Aren't there plenty more 308s within a few miles of you? Private may be cheap but buying from a dealer gives you a lot of legal redress when/if things go wrong.

My antifreeze mixture is colourless The orange/pink colourthat you expect to see is only a dye that is sometimes used as an indicator.

I run a weak mixture of antifreeze ie roughly 25/75 instead of 50/50 because the higher the proportion of water the better the cooling properties of the liquid

Does it really mean something if it blows?

It means there's blowby. All engines have blowby. Excessive blowby indicates engine wear. I don't think you'll be able to make even a subjective judgement by putting your hand opver the oil filler cap.

Absolutely true, Unthrottled: but given that most cars' original coolant is coloured (for whatever reason) what is worrying in this case is that a 2-year-old car doesn't seem to have the original coolant. If there isn't a plausible reason available for this, best to look for another car (even better, something othe than a Peugeot).

The point I was making is that cars are filled with 50/50 of a nice pinky/orange colour at the factory and that even after a number of years the coolant should still be clean and coloured. I was merely trying to illustrate this point. If there were problems (such as internal corrosion) then the coolant might be brownish and there might be brown stains in the expansion bottle.

A two year old car should have nice clean orange coolant in the bottle (whatever you might think of it's merits or not). The fact this car has clear water in there means the original coolant has been drained (or leaking slowly and only topped with water). On the presumption the current owner is the first owner then he should know why. I see nothing arbitrary in this - it suggests a problem.

When buying a secondhand car, it has to be judged by some critera. As clear water in the bottle is very odd for a two year old and there is no forthcoming explanation then unless this is a very special car, my opinion is to simply look for another.

As we can't see the car, all we can do is express an opinion based on the available information.

If the OP is especially keen on this car, he could buy it and flush out the cooling system with a proprietory compound and refill with fresh new coolant mix. It might be okay. He could use this to beat the price down a little. On the other hand................