You've seen numerous black and white photos of Comet 67P, which make the comet look grey.
Back here, we talked about how its gaseous emissions were a mix of ammonia, methane, formaldehyde, hydrogen sulphide, hydrogen cyanide, sulphur dioxide and carbon disulphide.
To a human nose, this would be combination of "rotten eggs, cat urine and bitter almond."
Now we know what the comet looks like to a human eye.
Here's the first true color image of Comet 67P, showing that when you cook those organic compounds in high-UV sunlight, they turn a reddish brown: