Turn the stained shirt inside out. Moisten the affected area by brushing it with a wet cloth.
Pour lemon juice into a small bowl. Dip an old toothbrush into the juice and then apply the moist bristles to the damp fabric. Spread the lemon juice over the stain until the area is completely covered.
Scrub the lemon juice into the shirt, working from the outside edges of the stain toward the center.
Place the garment in the sunlight and allow the fabric to dry. The combination of UV radiation and citric acid will break down the chemicals causing the discoloration. If it is a rainy or cloudy day, simply set the shirt aside. When the fabric is completely dry, toss the shirt into the washing machine.
Add ½ cup lemon juice to the wash water and launder the garment as usual. Remove the shirt when the washing machine stops and inspect the stained area; if any discoloration remains, repeat the procedure. Do not place the shirt in the dryer until the stain has been completely removed.