It's more complicated than you might think.
Old antifreeze was based on Methanol. You probably can’t buy it now and if you can, DON’T.
Methanol gave reasonable protection but being alcohol it evaporated quickly.
It is still used as an antifreeze agent in screen wash. Methanol does not damage engines or paintwork.
Ethylene glycol antifreeze was introduced during the late 1930s. It is a good antifreeze and doesn’t evaporate like methanol but is corrosive and toxic.
Thanks to its corrosive nature, manufacturers had to add corrosion inhibiters.
This eventually lead to the introduction of the standard; BS 580.
Methanol based products and products with poor corrosion protection didn’t meet this standard.
Ethylene glycol Antifreezes tended to be Yellow, Green or Blue.
To avoid methanol or a lack of inhibters you looked for one conforming to BS580.
The inhibiters were phosphate, nitrate, nitrite and silicate-based additives.
These antifreezes are still popular today.
The Volkswagon Audi group introduced an antifreeze type system using G11, G12, G12+ etc.
BS580 antifreezes are generally known today as G11 type.
Manufactures knew that ethylene glycol would pretty much last the life of the car. What they needed was corrosion inhibiters that would do the same.
It is corrosion inhibiter changes that have driven changes in antifreeze type. (They all remain based on glycol)
Many antifreezes use Organic Acid Technology often called OAT corrosion inhibitors.
The correct name for Ethylene glycol is Mono-ethylene glycol (MEG).
It has a density of 1.110 (Pure water is 1.0). Standard hydrometer antifreeze testers are designed to measure this.
Most OAT containing antifreezes are based on MEG and are classed as G12.
They are usually red. (but can be fluorescent yellow)
G12+ is a purple OAT based antifreeze.
In recent years mono-propylene glycol (MPG) antifreeze has been introduced as it is far less toxic. It contains the usual OAT corrosion inhibiters.
MPG has a density of 1.033 (MEG is 1.110). ).
Standard hydrometer antifreeze testers are NOT designed to measure this. They may not register at all or if they do the figures won’t be right.
You can purchase MPG (Propylene) antifreeze testers for this antifreeze.
The best way to get an accurate measurement is to use a refractometer.
How much antifreeze should you use.
Most vehicle manufacturers specify a 50:50 mix of water to antifreeze which gives protection down to roughly -35 °C.
60 - 70% antifreeze may protect down to -50 °C.
Never add more than 70% antifreeze as this will make protection worse, not better.
Can I choose by colour
NO, you cannot rely on the colour of antifreeze to tell you anything.
G11 Antifreezes tend to be Yellow, Green or Blue (Ethylene glycol) and use corrosion inhibiters based on phosphate, nitrate, nitrite and silicate. (not OAT based.
G12 antifreezes tend to be RED and can be OAT based or not and at least one G12 antifreeze is fluorescent yellow.
There are even some Green antifreezes that are OAT based.
So always read the label on the container.
Can I mix antifreezes.
Yes, all antifreezes can be mixed BUT If you do this the corrosion protection and the level of frost protection will be impossible to measure.
The high levels of Silicates in older G11 antifreezes can react with the inhibiters in G12 type antifreezes to produce a yellow/green sludge. So it makes sense to avoid putting G12 type antifreeze in a G11 filled car.
G12, G12+ and G12++ can all be mixed without problems. The only drawback is that topping up a G12 system with G12++ antifreeze may reduce the extra benefits from the G12++ antifreeze.
The AA recommend their technicians top up vehicles with a G12+ antifreeze such as Comma Extreme Red.
This particular antifreeze seems to cause few if any mixing problems.
Rule of thumb
Antifreezes have to be backwards compatible.
So, G12++ can be added to G12+, G12+ can be added to G12, G12 can be added to G11.
Avoid going the other way as it is not a good idea to put G11 in with G12 and avoid going too many steps back, E.g. G12++ into G11 is not a good idea.