Place popped popcorn in a greased pan in 350 F oven. Remove any unpopped kernels. In a large saucepan, melt margarine or butter over low heat. Add marshmellows. Stirl until completely melted. Remove from heat. Stir until smooth. Add red food colouring. Stir in popped popcorn, half at a time. Cool for about 10-15 minutes. Press mixture into greased heart-shaped pans. Cool again. Remove from pans and sprinkle with red coloured sugar. Place each heart in plastic wrap.