The parts of a fart we are capable of perceiving are primarily the methane (the part that makes it flammable and harder to breathe) various sulfides, ammonia, bacteria, and food residue (which creates the distinctive smell. And also some of them are flammable.)
Now, none of these have colors in their gaseous forms, but solid sulfur is commonly yellow, if you had an entire sea of methane, it would be even more colorless than water, but it might look slightly green at the depths of its white/gray appearance. Solid ammonia is pretty much just white and people only add stuff to make ammonia crystals turn colors. But I know what you're thinking. "I know farts smell bad, but if somebody's farts had that weird bitter burning smell of ammonia and/or sulfur, something must be wrong with them!" And that's true, although apparently they can smell that way if somebody is eating a protein-saturated diet. The stuff that comprises most of the smell, and, I daresay, the very character of the fart, is the dead bacteria and digested food. While other elements may comprise the gas, that's what creates the creates the shape that assaults the nostrils, tightens the throat and waters the eyes. That's the portion you think of when you think "fart". But what does that look like when it's solid?
Well, I'm pretty sure you know what that looks like when it's in visible form. Ladies and gentlemen, it's quite clear that the obvious answer was in front of us all along. Farts are brown.
