Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Line 20 small muffin cups or ramekins with paper liners.
Sift flour and baking soda together in a bowl.
Beat sugar, egg yolks, egg whites, and vanilla extract with an electric mixer in a separate bowl until very fluffy. Stir the flour mixture and soda in gradually; mix well. Place yellow, green, and red food coloring into 3 separate bowls. Mix about 4 tablespoons of batter into each bowl.
Fill the muffin cups 3/4-full with the original batter. Drizzle with the yellow, green, and red batters. Carefully place the muffin cups into the steamer. Place a clean kitchen cloth below the lid to prevent steam from dripping onto the cakes. Cook, covered, until tops spring back when pressed, about 20 minutes.
