Pre-heat oven to 350º F (static) or 325º F (convection). Spray bundt or tube pan with non-stick cooking spray and dust entire surface lightly with my gfJules™ Flour.
Combine the butter and your granulated sweetener of choice and beat well with your mixer’s paddle attachment, until the mixture is very light and fluffy (approximately 3-4 minutes). Add the eggs (or see egg substitute instructions in recipe) next, one at a time, beating well after each addition. Slowly add the milk, alternating with the dry ingredients, beating in between the additions. Beat just until smooth.
Divide batter into as many bowls as colors you are using. Return batter one at a time to mixer bowl and add food coloring, blending until desired color is achieved. Pour the batter into prepared (oiled and floured with gfJules Gluten Free Flour - recommended) pan, layering each color on top of the other. (Remember ROY G. BIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet = the colors of the rainbow, in that order!).
Bake bundt cake for 45 minutes, or cupcakes for approximately 25 minutes; test for doneness by inserting a cake tester or toothpick in the middle of the cake to be sure it comes out clean, with very few crumbs attached. Add time if necessary to fully bake the cake.
When done, turn off the oven and leave the oven door open to let the cake cool slowly there for 5 minutes or so, then remove the cake to a cooling rack.
After 15-20 minutes of total cooling time, gently invert the cake to remove prom the pan, then flip gently back onto the cooling rack.
Frost the cake only when fully cooled, or in the alternative, wrap the cake with wax paper or plastic wrap and seal inside freezer bags to freeze or refrigerate until ready to use.