Beat the eggs, sugar and baking powder at high speed until fluffy, then reduce the speed to low and add the oil and the milk, continue beating for a minute. Then still at low add the flour in two additions mixing well after each addition.
Stop the mixer and scrape out the sides and bottom and continue mixing.
Now divide the batter into two equal parts, add one part cocoa powder and mix well.
Assemble the cake in a greased pan by adding two spoonfuls of plain batter in the middle of the pan, gently tap the pan and let the batter spread. Next add two tablespoons of coloured batter on the top of the plain batter and tap the pan, again let the batter spread. Keep adding the batter in the same manner alternating between plain and coloured batter. At the end you might be left with some batter or each colour and find that you have no room to put it, just use the leftover batter to bake a separate small mini cake.
Bake the cake in a 350 degrees oven for about 30 to 40 minutes or until toothpick comes out clean.