You should treat removing stains, yellow or otherwise, from baby clothing that has been in storage in the same manner as stained vintage clothing. Time sets stains, and removal is often more complex than simply giving the stained area a squirt of laundry stain treater. Some brands of detergents work more effectively on established stains than others, and sellers of vintage linens and quilts swear by Biz and OxiClean. If the garment is white, you can rely on old-fashioned bleach and sun drying to remove the stain, but bleach breaks down fabric fibres, so you shouldn't use it on fine garments.
Check the care tag in the baby clothing before proceeding with the removal of yellow stains. If the garment requires dry cleaning, do not attempt to remove the stains yourself; take the baby clothing to a professional dry cleaner. If the garment is colour-fast, and you can wash and dry it at home, continue with the remaining steps.
Fill the bucket ¾ full with boiling water for all fabrics except synthetics and wool. For synthetics use hot (steaming) tap water; for wool use warm-to-the-hand tap water. Synthetic fabrics, such as nylon and acrylic, are man-made fabrics and react to heat differently than natural fibres such as cotton and linen. Many synthetic fabrics melt when introduced to extreme heat.
Add one cup of Biz to the water. Agitate the water with a spoon to dissolve the detergent.
Place the stained garments into the water. Soak white clothing with other white clothing. Soak coloured items separately with like colours even if the garment tag claims colour fastness. Boiling water can cause colours to bleed. Allow the stained baby clothing to soak overnight.
Rinse the baby clothing under the tap in cold water until all suds are gone.
Wash the baby clothing in the washing machine with your usual detergent on the machine setting appropriate for the type of fabric.
Dry the baby clothing in the dryer, or if the garments are white or light coloured, allow them to dry in the sunlight on a clothesline. Do not allow fabric to dry until you have removed the stain. Heat will further set the stain on the clothing.
You can use other detergents to soak the baby clothing, but Biz or OxiClean yield the most reliable results. Rubbing a paste of equal parts baking soda and water into a stain on baby clothing prior to soaking the garment in the detergent solution can give you extra insurance against the stain. Use an old, clean toothbrush to rub the paste into the fibres of the baby clothing. This is especially good on knit fabrics, such as onesies and T-shirt fabrics.
Use bleach on white clothes only as a last resort. Bleach can damage fabric fibres causing the fabric to break down during wash cycles and result in frays and eventually holes forming on the baby clothing.