Rainbow Cake. Italian Buttercream

The rainbow cake recipe (for 16cm diameter x 16cm height cake)

*** The vanilla cake:
150g butter (usalted, room temperature)
120g vegetable oil
450g sugar
1 tbsp vanilla extract

4eggs (55g each)
90g milk
200g Greek yogurt

500g cake flour
2 tsp baking powder
1/2 tsp salt

Food colors

***Italian buttercream
4 egg whites
180g sugar
45g cold water
500g butter (room temperature)