Preheat the oven to 325 degrees F, mist three 6-inch diameter cake pans with non-stick spray, and line with circles cut from parchment paper.

Place the cake flour, all-purpose flour, sugar, baking powder, baking soda, and salt in a large mixing bowl and whisk to combine.

Add the softened butter and mix on medium-low speed until the mixture resembles damp sand (about 1 minute).

Stir in the eggs, one at a time, and scrape the bottom and sides of the bowl with a silicone spatula.

Add the milk, Greek yogurt, and orange blossom water, mixing on medium speed for about 90 seconds to aerate the batter and build the cake's structure.

Fold in the Fruity Pebbles cereal, and transfer the batter equally between the three prepared pans.

Bake for 20 to 25 minutes, or until a toothpick inserted in the thickest part of the cake comes out clean or with a few moist crumbs.

Cool for 20 minutes, then remove the cakes from the pans and cool completely before filling and frosting with rainbow Swiss meringue buttercream.

In a large glass or metal bowl, combine the egg whites and sugar.

Set the bowl over a pot of barely simmering water, and stir continuously, until the sugar is completely dissolved. (Check by rubbing a small amount between your thumb and forefinger; it should be hot to the touch, and feel smooth, not gritty.)

Remove the bowl from the simmering water and whip on high speed until the meringue is thick, white, glossy, and can hold stiff peaks. (It should be cooled completely. There should be no hint of warmth when you place your hand on the side of the bowl. Refrigerate the meringue for 15 to 20 minutes if necessary.)

Add the butter in, a tablespoon at a time, beating well after each addition. (If the mixture begins to look curdled, continue to beat until it comes back together, before adding the remaining butter.)

Stir in the lemon extract and salt.

Place one Fruity Pebble cake layer on a serving plate, and top with buttercream.

Place the next layer on top, spreading on an even layer of buttercream.

Place the last cake layer on top and frost the entire cake with buttercream.

Scoop about 1/3 cup of frosting into 5 small bowls, and tint each one with gel paste food coloring in pink, yellow, green, blue, and purple.

Pipe horizontal stripes in each color around the cake.

Use an offset spatula or pastry scraper to smooth the icing and blend the colors.

Fit a pastry bag with a 4B tip and pipe remaining (untinted) buttercream in star shapes all over the top of the cake.

Garnish with gold sugar pearls, gold sparkling sugar, and gold disco dust.