Coolant Colours - What is the difference between red and green?

Is there a difference between RED and GREEN coolant?
This is one of the most commonly asked questions in the automotive cooling world and my view is the following:
OE manufacturer's state there is a difference but they make it difficult to ascertain what it actually is and this may be for commercial reasons rather than anything else.
What we do know is most coolants have varying freeze and corrosion protection.  They therefore have different levels of glycol and/or corrosion protection additives.
Some coolants like Red Tectaloy Complete Fill, technically speaking, is superior to their green coolant.  Tectaloy red coolant has a reagent grade indicator dye built into it.  If any corrosive acids build up in the cooling system for exhaust gas through head gasket issues the coolant will change from red to yellow.  Once a red coolant has changed colour it should be replaced as soon as possible because it has turned acidic and will be corrosive.

Why do we need several colour coolants?
The majority of cars some from the factory with fluorescent green coolant installed so the aftermarket tends to follow the standard set by automobile manufacturers.  Approximately 98% of vehicles use green coolant.
The main reason for various colours is to quickly identify which coolant has been used and to help avoid cross contamination of different coolants.

Is it ok to mix different colour coolants?
Mixing different brand or colour coolants is generally unsafe because the chemical makeup of two coolants is likely to be different and could cause major corrosion within the cooling system.

Is is ok to use coolant that has changed on the shelf?
No, the chemical composition has changed so it is unsafe to use (send it back to your supplier).

The correct coolant mix is an extremely important ingredient for the effective operation of a cooling system.  In a few cases it may be technically possible to mix green and red etc but the risk of creating a chemical disaster is far too high.  So in my view it is safer and cheaper in the long run to replace like with like, or use the manufacturers recommended coolant which should be precede by a very thorough flush.

The correct quantity (usually 50/50) of good quality coolant and the absolute removal of all contaminants and used coolant is the key to healthy cooling system operation.