One of the most common questions I get asked is how to get yellow underarm stains off from antisperperant.
The colour comes from the reaction of the aluminium in the antiperspirant. To get it off, you need a pretty strong acid, and a good choice is the sulphamic acid in BAM Easy Off bathroom cleaner.
An alternative, believe it or not, is asprin. Make a couple of tablets up in a cupful of water and soak on the stain before washing. The acetylsalicylic acid will complex the Al and break up the yellow colour