For gross Zombie teeth, a quick trick is to mix black food coloring in with mouthwash. Swish a little bit of it in your mouth and spit out the excess and you'll have nice minty fresh breath with really gross teeth. Make sure to be careful if you have dentures or veneers as it may cause some staining on artificial teeth. Another great addition to this look is eating a few Oreo cookies. If you get some stuck in your teeth it really adds to the gunky teeth effect.