Grey pus is bad. It makes me think it’s kind of necrotic where green means crazy, stanky bacterial infection.
Short answer: color is determined by the kind of bacteria causing the infection and the white blood cell system’s reaction to it.
Long answer:
I looked at some resources – white papers, drug trials, stuff from the FDA…the most regular-person-friendly info came from Wikipedia.
Before I go to the technical answer, I want you all to learn these phrases. I might get patches made. Or maybe a tattoo. “Ubi pus, ibi evacua!” “Carpe diem quam minimum credula postero!”
OK, on to color….Despite normally being of a whitish-yellow hue, changes in the color of pus can be observed under certain circumstances. Pus is sometimes green because of the presence of myeloperoxidase, an intensely green antibacterial protein produced by some types of white blood cells. Green, foul-smelling pus is found in certain infections of Pseudomonas aeruginosa. The greenish color is a result of the pyocyanin bacterial pigment it produces. Amoebic abscesses of the liver produce brownish pus, which is described as looking like “anchovy paste”.