For the cake:
1/2 cup (1 stick) butter
1 3/4 cup sugar
3 cups sifted all purpose flour
4 teaspoons baking powder
1/4 teaspoons salt
1 cup milk
5 eggs (separated)
For the frosting:
1/3 cup softened butter
1/4 teaspoon salt
1 teaspoon vanilla extract (or whatever)
1 pound (about 3 1/2 cups) sifted confectioners sugar
2 egg whites (unbeaten)
1 Tablespoon milk (about)
Step 1: Mixing the batter
Grease and lightly flour two 8 inch pans.
Cream butter and sugar well. Add dry ingredients, sifted together, alternating with milk and egg yolks, beginning with dry and ending with dry.
Beat the egg white until they hold stiff peaks and fold them into batter. (This step isn't completely necessary as when I added the food coloring I flattened all that fluffy goodness. I suppose you could separate the batter, then color it and add an equal amount of egg whites, but I went another way as you shall see)
Step 2: Coloring the batter
Find a glass or container that will fit a ziplock sandwich bag. Flip the rim outward. Separate the batter equally, about 1/2 to 1 cup should work. Add food coloring, close, and knead to mix.
Again, this takes away from the lightness of the cake, but doesn't bother the flavor one bit.
Do this however many times and colors you desire. I fudged on the purple so had only 5 instead of six. For yellow, I just left the batter alone, using only 4 bags for the color.