Yellow stains on denim come from a variety of sources. Some of the common causes include grass stains, sweat and paint. Simply washing the clothing does not always remove the yellow stains from the denim. Pre-treating the yellow spots beforehand helps ensure stain removal. Both baking soda and vinegar remove stains. Vinegar tends to work more effectively on organic and non-oil based stains. Baking soda removes other types of stains, but also works on organic types, as well.
Wet the denim with water. Use enough water to make the material wet, but not dripping with water.
Sprinkle a thick layer of baking soda over the yellow stain on the denim. Scrub the baking soda into the wet denim with the toothbrush. As the baking soda lifts the stain, it will gain a yellowish tint. Scrub for one minute.
Wash the denim item, with the baking soda still on the stain, in a washing machine as you normally would, Check the spot after washing to make sure the stain is gone. If it's still there, clean it again with baking soda.
Dry as normal.
Pour one cup of vinegar per item into a mop bucket.
Place the stained denim items in the bucket. Do not add more than three items.
Fill the bucket with enough water to cover the denim items. Let the items soak for one hour.
Remove the items. The yellow areas should appear fainter. Fill a spray bottle with vinegar. Spray the vinegar onto the spot. Wash the wet denim as normal in a washing machine. Check to see if the stain is still there. If it is, douse it with more vinegar and wash again.
Dry the item in a dryer.