Transmission fluid should be quite red in colour and very translucent. Any time it’s not, you almost certainly have something going on inside the transmission. When transmission fluid turns brown or black, that usually indicates wear inside the transmission from either being overheated or from what’s called slippage.

The bands and clutches that create the flow of power in the different gears of the transmission are covered in friction material that gradually wears out (like a conventional car’s clutch plates or brake pads). These bands and clutches get hot enough to either 'burn', or slip when they should grip. In the case of overheating, the transmission fluid gets 'cooked' as well, turning it a darker colour. And if there’s slippage inside the unit, the friction material on the bands and clutches can be shredded (like sanding a piece of wood) and the abraded material (sawdust) collects in the fluid, turning it black.

Overall, black transmission fluid suggests internal wear and that a change of fluid and a transmission service is required immediately. This should return the fluid to its correct colour, but you need to monitor it over time to ensure the same problem isn’t continuing.

Fluid will darken over time (as the bands and clutches wear normally) but if the fluid has suddenly gone from normal to black, then you very possibly have internal wear starting to make its presence felt.

Some Mitsubishi models in the past have had problems with a rough surface on the head-gasket sealing surface of the crankcase,. This has led to head gaket failures and coolant loss. This was fixed by about 2014 with a different machining process at the factory, so this could be your problem.

At the time, Mitsubishi issued a service bulletin for this problem and was modifying cars to fix them. However, whether this bulletin would still apply in 2025 is the big question. However, it would certainly be worth contacting Mitsubishi Australia’s customer service department to see if there’s still assistance available.

The problem could have been the coolant, rather than the cap. A higher-pressure cap can help if it's a minor problem but, in any case, if you run into problems in the future there is an admission of a problem by Mitsubishi.