Mix white cake mix, egg whites, oil, water and sour cream for 3-4 minutes on medium speed. Split cake batter into three equal amounts, adding red and blue food coloring to two of the portions. Coat three, 4 x 9-inch bread pans with cooking spray and lightly flour the bottoms. Add the three batters to the pans and bake at 350 degrees for 18-23 minutes. Allow them to cool and remove from pans.
Put a portion of the blue cake plugs back into the tops, covering the center filled with Pop Rocks. Cut licorice rope pieces approximately 1-1/2 inches long, and insert them into the centers of the blue cakes.
For maximum popping potential, pour the Pop Rocks into the cake centers no earlier than one hour before serving. The moisture in the cakes will cause the Pop Rocks to slightly snap and crackle on their own. In fact, they make a great sizzling sound on the plate. But after about 4 hours the Pop Rocks lose most of their popping power, so plan accordingly.