Why does sweat leave a yellow stain in your shirt?

The reason why there are yellow stains are because of body secretions called apocrine sweat and sebum, the oily secretion of the sebaceous glands. Sometimes it can be also due to deodorants.

These sebaceous glands are usually associated with hair follicles. Cells filled with fatty droplets die and burst and this provides lubrication for the skin and the hair. When the oils are exposed to air, they oxidize and a result it turn yellow. If you are not careful and do not remove it immediately, it can be permanent color. Also remember these seb. glands at the back of the neck cause ring around the collar and it is pronounced when you live in humid parts of the world such as South India.

The underarms and groin are rich in apocrine sweat glands which result in production of milky secretions and when acted by bacteria and it can have odor (which is called BO). Apocrine sweat contains many chemicals including the acidic substances that produce the characteristic underarm odor.

The biggest human sweat, produced by the sweat glands called eccrine glands uses this to regulate temperature. It is mostly water for cooling by evaporation and 1% of it contains salt, ammonia, uric acid and urea.