Yellow staining, like you have described your garments exhibit, is in fact a result of perspiration. Antiperspirant salts do cause yellow stains after repeated use. This happens due to a reaction between antiperspirant salts, sweet, sebum and detergents used to wash clothes. We would recommend our Shield Invisible range to avoid these marks.
To help remove perspiration stains on garments, you might like to try pouring a bit of warm clear vinegar directly onto the affected area and rubbing it into the fabric before placing the item in the wash. In our experience, the following actions have proven to be useful to remove older, more stubborn stains:
If the manufacturer's washing instructions allow, a thick paste of citric acid crystals (one cup) mixed with liquid detergent (half a cup) and carefully applied directly to the affected area of the garment with an old toothbrush usually works. We recommend you keep the garment damp during this process and leave the paste on for several hours or, if possible, overnight.