Yellow stains in your microwave are likely caused from food residue such as grease or butter that's burned onto the microwave's interior surface. Even if you set the microwave time accordingly, it is possible for the food to burn inside the appliance and leave behind a stain. Usually you can't remove yellow microwave cooking stains simply by wiping down the interior with a wet rag. Take a more aggressive approach to remove yellow microwave stains for good.
Make a solution of hot, soapy water. Dampen a rag with the soapy water solution and wipe down the inside of the microwave to remove the surface grime. Rinse with a rag and clean water, then dry thoroughly.
Dampen a nylon scrubbing pad with hot water. Sprinkle baking soda directly onto the sponge.
Scrub away the yellow burn marks with the scrubbing pad and baking soda. Rinse out the sponge and apply more baking soda as needed.
Rinse the microwave with a damp rag to remove cleaning residue. Dry thoroughly with a clean rag.