The old original type which I grew up with was usually blue but occasionally green. These were Silicate based and good for about 3 years. Then (maybe 15 years ago?) OAT (Organic Acid Technology) products were introduced and the majority of vehicles on today's roads are probably running on this - It's usually a reddish orange colour and is recommended to be changed every 5 years. My relatively new 2016 Ibiza runs a purple fluid which I think is the latest HOAT (Hybrid Organic Acid Technology).
Until quite recently I tended to think, if it's blue I'm looking at the "old" silicate stuff. If it's red/orange it's OAT and now I have come across this Purple stuff. I know for sure, as Eklipze states above, silicate and OAT do not mix. I suspect you might get away with mixing OAT and HOAT but probably to the detriment of the product so why would you? I bought a litre of genuine VAG purple fluid for topping up the Ibiza
In addition to the above I have also seen green and yellow coloured products and I recently discovered that the blue antifreeze used in Hondas (type 2) is an OAT formulation (I think? It's got a 5 year change recommendation, so must be OAT?) This worried me so much that I bought a litre of genuine Honda product to top up the Jazz in our family as I didn't want any mixing issues even though my local factor recommended his RED OAT generic product I know of one other maker for sure who recommends the Red OAT as long as you know for certain that the blue stuff in the rad is definitely Honda type 2! It'll be getting changed to red OAT next service. The Honda stuff is extortionately priced.
So where am I going with this? I suppose what I'm saying is that, although I used to think you could identify coolant type by colour, I no longer think this is a reliable way to do it. I really don't buy the old silicate stuff any more. If we buy a new car to join the "family fleet" one of the first jobs is to drain and flush the cooling system - doesn't take long and often you will find a dodgy hose, water pump, etc while you are doing it. If the history of the car is not known or unreliable, a timing belt (and often water pump) can be done at the same time. The situation you mention here "Old fluid reddish in colour but may be due to dirt" Well the likely hood is that it's going to be OAT (most of the factors and other suppliers sell OAT more than anything else now) but how long has it been in there and is it a mixture of silicate and OAT? The very important additive package in the coolant - which performs a multitude of protective tasks not least of which being to reduce corrosion, and in particular electrolytic corrosion (ally/cast Iron/steel interfaces) - may well be depleted? So, flush it well out (I stick the garden hose in with the hoses disconnected and let it run through, including back flushing the heater matrix, for a few minutes until it runs absolutely clear.) Then I go on the Opie oils website (others are available) but I trust them - and put the reg no in whereupon I get a recommendation for what I need. Sometimes I buy from them and sometimes from other sources but within a month of buying the vehicle I will have done a front to rear check over the whole vehicle with new correct spec oil and coolant and, probably a timing belt and water pump (unless there is good evidence of the belt fitted being relatively new) I'll also have started a service sheet for it with notes about brake pad/lining condition, brake hydraulics (ie cylinder/caliper condition/leaks, pipe corrosion and flex hose condition, ball joints, suspension bushes, rack and it's gaiters, tyre wear, etc, etc). I went through a "phase" of using distilled or deionised water to fill (50/50 mix with concentrated antifreeze) but our water up here in Edinburgh is so soft that I am not now so obsessed by this. If I lived with my daughter down near Salisbury Wilts, having looked in her kettle and having to drink the disgusting cup of tea it makes, I would be using the distilled stuff. Halfords sell 5 litre bottles of the stuff for relative peanuts.
By the way, If you decide to do a complete flush through and change, please drain as much of the old coolant into a container and dispose of it responsibly (my local recycling centre takes both old engine oil and coolant). I believe coolant is very toxic (from what I've read) and should never be simply poured down the drain.
