First preheat the oven at 180°c. Mix flour, baking powder and salt together, keep aside for later use. 
Add baking soda to beaten yogurt and give a quick stir and keep it aside for 8-10 mins or till it become nice frothy. 
Now mix vanilla essence, sugar and oil together till sugar dissolves and mixture become smooth and fluffy.
Don't over mix otherwise all the bubbles will sink. Mix dry and wet mixtures together and fold to get lumpless smooth batter .  *** Batter should neither too thin nor too thick. If needed add some milk/water , little at a time to get proper consistency.
Now divide the batter in four equal parts ,add cocoa powder to one , orange and blue colour to other two and leave fourth one as it is. Mix well but don't over do.
*Add some milk/water in Cocoa batter to get same consistency of all the batters.
In a greased baking pan , lined with butter paper , pour one big spoon of one batter and tap the pan to spread .then pour one big spoon of other batter over it and again tap the pan and do it alternatively till batter finished.
**Now using a knife or skewer make 6-8 lines on cake starting from center of cake towards edge, at equal distance and cleaning knife/skewer every time.
Now again makes lines, in between these lines but in reverse direction, from edge of cake towards center. This will give a nice pattern on the cake.
This step is completely optional, you can skip it without getting any change in final product.
Then bake it for 50-55 mins. But keep an eye after 40 mins on it as baking time may vary oven to oven. I baked on convection mode for 55 mins. 
Check it with knife or skewer by inserting in middle of cake ,for doneness. If knife/skewer comes out clean then cake Is done otherwise bake for some more minutes. 
Let it sit for 5-10 mins then invert on wire rack and let it cool completely.
Its so soft and moist cake that you cannot have just one piece.  *You can also use half or 1/3 the quantity as I made bigger cake for kids sharing day.
