I must say I am not a fan of food coloring but it is a rainbow cake! Trying it once in a life time :) You can use an vanilla cake recipe that you have or ones that are already in a box. If you do not have one, this is a recipe from joy of baking.
1 box vanilla cake
Food coloring (yellow, blue, green and red), to make more colors have some fun and mix colors (for example: red and yellow, red and blue.)
1 cup whipping cream
Walnuts for garnish
1) Make the cake according to the recipe you are doing. Divide the cake batter into small separate bowls equally.
2) For each bowl add a separate color or mix two colors, mix well to have a uniform color.
3) You can use any pan you wish I used two 20 cm round pans. In each pan divide the batter of each color equally (if you are using one pan then no need to divide, just spread on top of each other).
4) Bake the cake as the recipe suggests or the instructions.
5) After the cakes have cooled. Add any color if you wish to the whipping cream then beat it. Spread apricot jam on the top of one cake (if you have one cake you can cut it in half) then some of the whipped cream. Put the second cake on top. Spread the rest of the whipped cream on the assembled cake and garnish it. Enjoy.