Beat margarine, sugar, and vanilla until frothy. Add eggs one at a time.
With a wooden spoon, incorporate flour and baking powder alternating with the buttermilk.
Mash raspberries through a sieve, collecting the juices. Throw away seeds.
Divide dough in three different bowls. Mix in food colouring to one of the bowls. Mix cocoa with milk and add to another bowl. You should now have a bowl of each plain, pink, and brown dough.
Preheat oven to 350°F.
Using a tablespoon alternate colours, dropping them by the spoonful on the bottom of a spring-form pan. Pour raspberry coulis into the joints where the colours meet. With a fine stick or knife "stir" minimally to give a marbled effect.
Bake 1 hour. Wait 5 minutes before turning over on a wire rack to cool.