Drain 1 (1 lb.) can pitted red tart cherries, reserving liquid. Add enough water to cherry liquid to measure 1 cup. Mix 1/2 cup sugar and 1 tablespoon cornstarch in small saucepan. Stir in the 1 cup liquid. Cook, stirring constantly, until mixture thickens and boils. Boil and stir 1 minute. Remove from heat, stir in cherries and 4 drops red food color. Cool thoroughly.