Yellow markings on a toilet seat can be due to urine stains, chemicals in cleaning supplies, hard water—even sunlight. To remove these stains, take off the seat and soak it in a solution of 1-gallon water mixed with 1 cup of bleach. After about 5 minutes, scrub the stains until they disappear, then rinse the seat under hot running water and dry thoroughly. Reinstall the seat, taking care to ensure that it's secure.