Remove yellow underarm stains from white shirts by applying a paste made by mixing equal amounts of baking soda, hydrogen peroxide and water. Protect the counter underneath with a folded white towel, and allow the mixture to soak for at least 30 minutes before laundering the garment. Only use this method on washable fabrics.
Treat the underarm stains on colored materials by adding 1 cup of white vinegar to the washing machine and filling it with cold water. Soak the garments for at least 30 minutes before washing in the normal cycle for the removal of stiffness and odors.
Treat any stains as soon as you notice them starting to form. If you do not have time to treat the stain when you first notice it, avoid drying the garment in an automatic drier. Air dry the garment after washing and treat it as soon as possible to prevent setting the stain.
Most underarm stains are due to a reaction of perspiration and aluminum containing antiperspirants. Switching to another antiperspirant may eliminate the formation of these stains. Applying the product a few minutes before getting dressed or directing the warm air of a hair drier toward it so it is dry before getting dressed helps to eliminate the stains.