Coolant colour
This is probably the most widespread misunderstanding about coolant. Colour is dye and does not determine chemistry. One manufacturer may sell the same chemistry in two colours, while two different manufacturers may sell entirely different fluids in the same colour.
Then why must you not mix?
Because although colour does not determine chemistry, the chemistries genuinely do differ, and their inhibitor packages frequently neutralise one another.
Mixing a phosphated Japanese fluid with a silicated European one, for instance, can produce a precipitate that resembles neither and performs the function of neither. The result is sludge, blocked passages and the loss of both protection packages.
Honda's manual states this directly, that blue Type 2 must not be mixed with green Type 1, pink, orange or yellow.
Colour is still useful, but for a different reason
Colour is information. At a glance it tells you which family of fluid is probably in the system, which is useful when you do not know the car's history.
It is also a leak-diagnosis tool: a blue puddle in the car park tells you immediately that this is coolant and not oil or brake fluid.
That is why our advice to Toyota owners is what it is. If the system holds pink, it is better to refill with pink, not for chemical reasons but to preserve the information.
A short orientation
This is a general orientation, not a rule. Always defer to the manual.
- Blue. Often an Asian HOAT with phosphate: Honda Type 2, Nissan LLC, Subaru, Hyundai and Kia.
- Green. Either an older IAT type or the green variant of an Asian HOAT. Be especially careful here.
- Pink and red. Often Toyota SLLC or an Asian red HOAT.
- Orange. Often a Dex-Cool style OAT, usually containing 2-EHA.
- Yellow. Often “universal” fluids whose composition varies widely.