IK, make that a solution of 3% peroxide, no more --- or you might weaken some fibres and also be left with slightly blistery skin that is dashed uncomfortable as it peels. But yes, it works, on most natural fibres certainly.
Incidentally, it is also about the only thing that deals with mildew marks decisively. (No, don't ask; where I live, I get bracket fungi on my books in a bad month!)
Other options are baking soda (tad harsher, but good for some stubborn synthetics); vinegar (likewise); and in an emergency (say in a hotel room without ready access to cleaning stuff), try an aspirin!
Una, you have a point --- many antiperspirants contain ingredients that themselves stain yellow or set stains. That said, apocrine sweat (the kind coming from the underarm and groin) IS a different chemical composition to regular sweat elsewhere on your body, and varies just as each person's hair and skin moisture levels, sebum levels etc vary. Some people, unfortunately, seem to sweat more 'yellow'
Which reminds me of a personally untried 'remedy'. Apparently you can also deal with the stains by soaking in salt water --- the idea being to mimic sweat, which dissolves the stain. I'm not entirely sure of the chemistry here, I have to say! Could well be an old wives' tale... :-/
