What do the Different Colours of Antifreeze Coolant Mean – and Can You Mix Them?

Antifreeze, coolant, engines, reservoirs, and tubes – what does it all mean? Browsing through the coolant section of an auto shop can often feel like walking past a rainbow: what do all the colours mean?

Well, these days, they don’t actually mean much – but they do have a history, rhyme, and reason.

Let’s look at it.

What is antifreeze and what is coolant?

While they often go by different names, antifreeze and engine coolant are the same thing. They should really be called “engine regulant” or something official-sounding like that because their job is twofold: to stop engines getting too hot in the summer, and to stop them getting too cold in the winter.

You’ll often hear of high-tech things being “liquid cooled” – which sounds super fancy but is actually just a case of pumping water-based coolant through pipes in order to warm the water up and take that heat away from the thing you want to cool.

Now, water’s great at taking on heat and giving heat off, but it also has some undesirable properties when it comes to cooling mechanics. Namely, it freezes in low temperatures, boils at a relatively low temperature, and often speeds rust & corrosion in mechanical parts.

To this end, various antifreezes have been formulated to try to combat these problems with different aims and different levels of success.

How does coolant work?

Without going too much into the science, water is mixed with “an antifreeze” – which means a chemical agent that increases the boiling point of the water. Increasing the water’s boiling point also decreases the point at which it freezes because it develops a greater capacity for taking on and transferring heat. The idea here is to make sure that the coolant stays in its liquid form – because if it freezes it’ll expand (breaking any rigid mechanics in the pump).

There are many variations of additives, but the most common ones are ethylene glycol and propylene glycol.

Can you use tap water as coolant?

As an absolute last resort, you could – but it’s strongly advised that you don’t. The tap water in most parts of the UK contains a lot of chalk – and this is likely to clog up your coolant system and need lengthy repairs or a complete overhaul.

Your best bet is to get into the habit of having spare coolant, engine oil, and screen wash in your car boot at all times. But running an engine without any coolant at all is likely to cause real trouble. So if you’re caught short and have to use water, opt for distilled water rather than tap water. Be sure to keep an eye on your engine temperature and drive slower to stop your engine from overheating.

So, how about the colours?

Originally, the different colours of antifreeze were intended to make clear which particular type of antifreeze was currently in the vehicle. That’s because, due to the different chemical make-ups of different antifreeze mixtures, mixing different ones is likely to cause unwanted chemical reactions between the two.

You don’t need to worry if you have already mixed two, it’s not going to cause any noxious or explosive reactions, but is likely to become a thicker mixture that’s more likely to clog up your cooling system.

The main colours are:

Green antifreeze

Green coolant is most likely an old formula that uses Inorganic Additive Technology (IAT). This was the first form of antifreeze developed and was used until well into the 90s. It’s called “Inorganic Additive” technology because it uses inorganic materials to achieve the antifreeze and anti-corrosive effects: specifically, phosphates and silicates.

It usually needs changing around every two years as well as checking monthly or before every long journey.

Orange antifreeze

With growing use of metals like aluminium and nylon in car cooling systems – they’re really strong while also lightweight – the original mixtures of antifreeze became ineffective at stopping corrosion within the system. So, auto companies developed antifreeze using organic acids (though neutralised so they don’t burn through anything) as corrosion inhibitors and azoles as antimicrobial agents.

These are usually orange or red and need replacing around every five years in addition to checking monthly or before each long journey.

Red antifreeze

While this is also often orange, red antifreeze is usually a Hybrid Organic Acid mixture that combines benefits of both other types. It is also able to last up to five years but should be checked monthly or before each long journey.

Which colour antifreeze should I use?

The original colours were used to distinguish easily between two main types of coolant, but now don’t actually mean very much because there’s no regulation on them. With so much variation in available coolants these days, don’t rely on the colour of what’s in your reservoir to make a guess. Check your car’s maintenance manual for what’s required.

If you’re unsure, head to your local Kwik Fit centre, where our experts can help you in no time.