Why are my armpits yellow?

Understanding Yellow Armpit Stains

Your perspiration is primarily made up of water, along with ammonia, urea, salts, and sugar, and it is naturally clear and scentless. However, when your sweat interacts with certain substances such as the active ingredients found in your antiperspirant, laundry detergents, or bacteria, it can develop a yellow hue, resulting in persistent yellow stains.