Now divide this mixture into three equal portions.
Mix cocoa powder with 2-3 tbsp of milk and blend it with one portion of the divided mixture.
Blend the red food color with another portion of the cake mixture.
Now coat a baking tray with butter and pour the red colored cake mixture first in the tray, then the plain cake mixture and finally the cocoa powder mixture on the top.
Take a toothpick and simply draw lines in the cake(use your creativity here, you can line it randomly or spirally..it¦s up to you).
Bake it for 1 hour in 160c. Tri-color marble cake is ready.