How To Get Rid of Yellow Armpit Stains--- 1 tsp dawn, tsp hydrogen peroxide, TB of baking soda. Mix together. Use small laundry brush and scrub it in. Let it stand for approx 1 hour and then launder as usual. ( instructions copied from this pin)
How to Remove Deodorant Stain Buildup on Dark Clothing
MS: DIY Underarm Stain Remover: 1 part Dawn (or any dishwashing liquid, I assume), 2 parts hydrogen peroxide, then add baking soda to create a paste. Use brush to work it in, and let it sit for 1 hour.