Mash the cooked salmon and gradually add the milk; alternatively put the cooked salmon and the milk in a blender or food processor and process until creamy.
Stir in the margarine or low-fat spread, add the food coloring, and beat vigorously until stiff. Put in a glass bowl or a mold so that the mixture fills it by three-quarters.
Chill for 20 minutes, then decorated with the cooked shrimp, and pour on just enough heated gelatin to cover them. Once this layer has set, add further gelatin to taste and leave for an hour in a cool place or the fridge. To serve, turn the mousse out onto a plate and divide into portions.