Human wetware is astonishingly good at pattern recognition and interpreting complex, noisy data, but it's also painfully buggy. Today, the internet came to a screeching halt debating a dress's colour, but the same optical illusion is responsible for how uncertain we are about interpreting colours on Mars.
When we send robotic explorers to Mars, we equip them with colour calibration targets of known optical properties. Yet, even with those calibrated instruments, the view of the same rock in "true" colour can change dramatically just based on the brightness from the time of day.
This leads to an almost philosophical problem: even raw images don't tell us what we'd actually perceive in different lighting conditions. The problem gets even weirder when you try to compensate for lighting that we're familiar with instead of the lighting as it actually is on the planet. Here's Mount Sharp in true colour, and then rebalanced to blue skies to look like it would under more familiar Earth-normal lighting:
Mount Sharp in natural Martian lighting. Image credit: NASA/JPL
Mount Sharp rebalanced to Earth lighting. Image credit: NASA/JPL
This illusion hit the earliest observers, too — it's why Percival Lowell thought he saw canals, and why that myth persisted right up until planning the first up-close mission to the planet.
Like finally seeing that dratted dress as black and blue, trying to imagine how we'd see the planet if we had real, human eyes there is not as simple as it seems like it should be!