Why does some underarm sweat turn shirts yellow?

There are two types of sweat glands: eccrine and apocrine. Eccrine sweat glands are found all over the body and are particularly abundant on the palms of our hands, the soles of our feet and on our forehead. They mostly secrete water with high concentrations of salts. Apocrine glands are mainly concentrated in our armpits and around our genitals, and rather than just secreting water and salt, they also secrete lots of fat and protein. This makes the sweat thicker, more yellow, and produces the stains we find on our nice white shirts. Our armpits are a seething mass of bacteria and those bacteria are growing on the sweat we produce. The bacteria break down the fats and proteins to make rather niffy compounds, which is why your armpits can smell especially bad even though you've been sweating all over. Another possible cause of the yellowing is that when bacteria go about their metabolism, some of the things they produce are quite acidic. Many dyes in clothes are fixed by various acid or alkali processes. I'm thinking that perhaps some of the acid that's produced in the armpit is sufficient to affect some of those dyes.