Shake the bottle of theatrical tooth paint thoroughly. You can find tooth paint at costume stores or online by searching for "tooth paint."
Place your fake teeth on a piece of paper towel. Remove the top of the tooth paint bottle. You'll notice a brush is attached to the top like a nail polish bottle.
Scrape the brush against the inside of the bottle's lid, lightly, to remove excess paint from the brush.
Brush the color onto the tooth or teeth that you'd like to decorate. Cover the tooth thoroughly with the paint so that it completely changes color.
Allow the teeth to sit until the paint is dry to the touch -- about 5 minutes -- before adding a second coat, if necessary.
Scrape the teeth with your fingernail or a toothpick to remove the paint when finished.