I often wondered and previously blogged about the dilemma of existing yellow stains on armpit areas of white shirts. From research, I've found that the yellow stains come from the aluminum in the deodorant that we wear which accumulates over time. The stains get worse if the shirts are washed in Clorox or placed in the dryer; the best way to prevent the yellow stains is to not wash the shirts in Clorox and instead of tumble drying clothes, air dry to prevent set in stains.
When it comes to removing yellow stains on white shirts:
1) Take baking powder and sprinkle some on a white cloth
2) Add a little water and rub on the yellow spots and let sit for an hour
3) After 30 minutes of the hour wait, put peroxide on the white cloth and rub on the spot(s) aswell.
4) after the hour is up, rinse the spot(s) off in cold water and let air dry
After the shirt(s) dry, yellow stains hould be gone. Peroxide and baking soda is a great whitening agent for clothes. In addition, 100% cotton white shirts are much easier to clean than other fabrics so remember to check types of fabric before purchasing future white shirts.