Starter Challenge: Colours in nature.
Dammit, I took the perfect photo for that two days ago. There was a particularly bright (double, as it happens) rainbow and I was trying to make sense of why there was a green band inside the blue one, which wasn't a feature of rainbows that I'd noticed before.
Needless to say, it wasn't present in the recorded image. I assume that this was due to my wonky cone cells doing the thing that causes people with non-wonky cone cells to perceive a purple band inside the blue, and getting it hilariously wrong.