Refrigerate three colours of batter until they are ready to bake. Pour the remaining three colours into the oiled cake pans, then bake for 15- 20 minutes or when you poke a skewer into the middle and no batter sticks to it.
Cool the three cakes on a cooling rack. Wash the cake pans then bake & cool the remaining three cakes.
Make sure the cakes are completely cooked. This is very important or else the fosting will melt.
Starting from the bottom colour (purple), frost the top of the cake layer with Betty Crocker's vanilla frosting. You can decide how much frosting you want in between each layer, but I prefer a thin layer because the frosting is already really sweet.
Repeat with blue (2nd last), green (3rd), yellow, orange and then place the red layer on top without icing.
If your cakes are too "puffed up", then there will be a large gap in between each layer on the outside of your cake. This means that you will have to add a lot of icing to fill those gaps.
You can cut those edges off to make the outside surface flat. Compare the picture above with picture below, the cake's outer layer is trimmed.
Outer layer trimmed
Spread a first light coat of frosting on the edge of the cake to seal in crumbs, then spread a second and final layer of thicker frosting on the cake. Smooth it out with a larger, flat knife.
Spread frosting on the red top layer of the cake, again smoothly.
Now, decorate the cake with whatever you want. Add hundreds and thousands on top and to the sides, mix the food colouring with leftover icing then using a piping bag, make figures or shapes on the top of the cake.