Question: What color are farts?
Short answer: Green.
Long answer: We received this question from a reader, and we have to admit that it didn’t make a lot of sense to us initially. Although people with synesthesia often associate colors with sounds or smells, most people are only able to see farts under extreme weather conditions.
However, after further discussion we were able to arrive at an answer. The key fact is that hydrogen sulfide, the main constituent of fart smell, absorbs light in the ultraviolet range (Zhang et al., RSC Advances, 2017).
This means that less ultraviolet light would be reflected back to an observer who was looking at a fart.
Color vision can in fact be quite sensitive to a lack of particular wavelengths, and this is the basis for color-opponent theory, first developed by Ewald Hering in 1892.
The idea is that certain pairs of colors are treated as opposites by the retina, with the main opponencies being red vs. green and blue vs. yellow. So a lack of red is seen as evidence for green, and a lack of blue is seen as evidence for yellow. This peculiarity of the mammalian visual system is responsible for a number of interesting color aftereffects.
So what is the opposite of ultraviolet light? In all likelihood the human visual system is not very sensitive to ultraviolet, but other animals make good use of it, including butterflies, reindeer, and hedgehogs. Mice use ultraviolet light to detect predators, and in these animals, there is a clear opponency between ultraviolet and green sensitivity (Qiu et al., Current Biology, 2021).
Thus the answer to the question is that farts will appear green, at least to a mouse, and probably to a butterfly as well.