Mix 4 cups warm water with 1 tsp. liquid dish soap in a bucket.
Apply the soapy solution to the walls with a clean sponge.
Scrub the wall gently with the sponge until it is completely clean.
Wipe up the soapy solution with paper towels. The yellow stains may be gone if they were slight or were not on the wall for long.
Mix 1 cup bleach with 1 gallon water in the bucket. The bleach will soak into the wall to remove the stain.
Apply the bleach solution to the walls with a clean sponge.
Let the solution sit on the walls for at least 10 minutes to give it time to loosen the stain.
Scrub the walls with a soft-bristled brush until you've removed all of the yellow stain.
Repeat the bleach process if the stain is still visible.
Rinse the walls with a sponge soaked in clear water.
Dry the walls thoroughly with paper towels.