Wash and dry apples. Remove stems and insert skewers. Combine sugar, cream of tartar, salt, and water in a deep saucepan. Add food coloring to get desired color. Place over heat and stir until sugar is dissolved. Cook rapidly without stirring to medium crack stage (290 degrees F). Do not stir while boiling. Remove from heat and immediately dip apples. Twirl to cover, and place, stick up, onto waxed paper to harden.