Lift underarm and collar yellowing out of white cotton with a baking soda and peroxide paste, no harsh bleach required.
In a small bowl, combine 1 tablespoon baking soda, 1 tablespoon hydrogen peroxide (3% from the drugstore), and 1 teaspoon dish soap. Stir into a spreadable paste. Avoid chlorine bleach on yellow underarm stains — it reacts with the residue and turns them darker.
Lay the shirt flat and scrub the paste into the yellowed area with an old soft toothbrush, working in small circles so it penetrates the fibers. Cover the whole discolored patch, not just the center.
Leave the treated shirt in a warm spot or in indirect sun. The peroxide needs time to break down the protein and antiperspirant buildup. Don't let the paste dry completely — dampen it with a spray of water if it starts to crust.
Launder the shirt as usual in the warmest water the fabric allows. Skip the dryer the first time — heat sets any remaining stain. Air dry and inspect; if a faint shadow remains, repeat the paste treatment before the shirt ever sees the dryer.