Yellow underarm shirt stains are likely caused by a combination of deodorant and perspiration. Many deodorants and antiperspirants contain aluminum salts. When these salts are combined with laundry detergent, especially in cooler water settings, they are not easily dissolved, and they remain on the fabric.
To remove the stains, try soaking the shirts in warm water with an enzyme pre-soak product or rubbing the soiled area with white vinegar. Wash in the hottest water safe for the fabric. If the stain remains, dampen and sprinkle stain with meat tenderizer. Let stand for about an hour, and launder again.
To avoid new stains from forming, always wash the shirts in the hottest water safe for the fabric. Also, allow antiperspirant to dry completely before dressing.