i made this rainbow cake with toba garrett's moist yellow cake, http://www.epicurious.com/recipes/food/views/Moist-Yellow-Cake-109358
i love this cake plain, but the food coloring takes away from the great flavor. i tried to put some back by adding two extra teaspoons of vanilla.
the whole cake pictured was a practice cake. the slice is what i ended up making for the party.
these are my notes:
each layer is about 0.5" tall.
~240 grams batter per 8" round pan
baked 4 pans, ~12.5 min
baked 2 pans, 12 min
i used americolor gel colors
super red (3 drops) + red red (1 drop)
electric orange (3 drops)
lemon yellow (3 drops)
leaf green (3 drops)
royal blue (3 drops)
violet (3 drops) (better than regal purple)
the practice cake was iced with Whimsical Bakehouse House Buttercream. i wrote in my notes that a half-recipe (~4.25 cups) is enough to fill two 8" 6-layer rainbow cakes. i'm not sure if that includes icing the cake as well. (sorry, it's been over a year since i made this cake.)