Like any other kitchen appliance, the microwave is exposed to stain-causing substances on a regular basis. Grease and food splatters can build up to create stains and discolored areas in your microwave, and some stains can become permanent more quickly than others. Wiping your microwave dry just about every time you use it is a preventive measure.
If your microwave interior has discolorations on the plastic that are essentially erratically shaped yellowed areas of the plastic interior casing, they are likely grease stains. Such stains form when grease or food cooks onto the surface. Your prospects for removing such stains are limited.
Another form of yellow stain might be something less permanent, such as curry, turmeric or mustard items, any of which would cause a yellow color transfer stain. You should be able to wipe these clean if you do so before turning the microwave on.
Pour water into a 4-cup microwave-safe bowl to fill it 3/4 full. Add 1/2 chopped lemon to the water. Place the bowl of water and lemon in the microwave.
Set the microwave on the "High" power setting. Microwave the water and lemon solution for five minutes to bring it to a boil.
Leave the steaming bowl of lemon water in the microwave for 10 to 15 minutes so it can cool. As the steam escapes the bowl it softens stain-causing materials inside the microwave.
Remove the bowl from the microwave. Pour white vinegar over a sponge to dampen it. Wipe down the inside of the microwave to remove yellow stains.
Sprinkle baking soda onto the sponge to scrub off stubborn stains.
Rinse the surface of the microwave by wiping it off with a damp cloth. Dry the microwave with another cloth.