You can try Carbona Stain Removers. They are sold at most grocery stores as well as Wal-Mart, KMart & Target. They come in a little yellow bottle about 4-5 inches tall. They make a different formula for each kind of stain: i.e. grass stains have a different formula than ketchup stains. And you guessed it, there's one for underarm stains! It works, but I do find that after a few wears & washes, the stain comes back & you have to repeat the treatment.

Pour white vinegar on them.Then wash normally. Works for me. Cheaper than a "specialty" product and easily found in all agrocery stores.

My DD makes a "paste" of powdered Oxyclean and a little water to remove those horrible stains from her DHs undershirts. She lays the shirts out on her kitchen counter and spoons a thick coating of the paste on the shirt underarms and leaves them to soak for 4 to 6 hours. Then she washes them and the stains come out. I don't know how the bleaching agent would affect colored clothes.

I don't know if those yellowy stains are from salts in our sweat or from chemicals in deodorants/antiperspirants. My Grandmother had a laundry secret for whitening. She laid her whites to dry on the grass on a sunny, not windy, day. The combination of sunlight (ultraviolet light) and grass (chlorophyll) seemed to get everything white. Just watch out for bugs.

And bird poop, especially if there are blackberries growing close by.