Lemon juice
The acidic properties of lemon juice are very effective for whitening whites. Apply a few drops of lemon juice to the stain and leave it to act for an hour, with the head exposed to the sun. Then wash the garment normally.
Remember that not all fabrics are the same and some are more delicate than others. Always read the label on the garment to understand which washing method is most appropriate and whether it is possible to apply products such as bleach or baking soda.

Hydrogen Peroxide
Hydrogen peroxide is very effective against yellow stains on white clothes, but it is only safe to use on cotton, synthetic or nylon garments.
Pour two equal parts water and hydrogen peroxide into a bucket and soak the garment for a few minutes, then wash as normal.

Active Oxygen Stain Remover
There are several options on the market, but active oxygen stain removers are generally a quick and effective way to get rid of white stains, not just yellow ones.
Read the instructions carefully so you can apply these stain removers without risking damaging your clothes.