Place popcorn in a greased 13x9-inch pan and keep warm in a 300 degree F oven while making marshmallow mixture (remove unpopped kernels).
In a large saucepan melt margarine over low heat. Add marshmallows and stir until completely melted. Remove from heat. (Or in a microwave-safe bowl, micro-cook margarine and marshmallows on high for 1-1/2 to 2 minutes or until margarine is melted and marshmallows are softened, carefully stirring once. Stir until smooth. Stir in red food coloring. Stir in popped popcorn, half at a time. Cool until easy to handle (10 to 15 minutes).
Press mixture into greased individual heart-shaped pans. Cool. Remove from pans. Sprinkle with red sugar. Makes 9 hearts.