To get the royal blue velvet color, remember the toothpickful of violet paste food color; if forgotten, the cake will bake up turquoise in color. Use paste food color to achieve the royal blue velvet color. If you use liquid or gel food color, your cake color will vary.

1 box white cake mix
1 1/4 cups buttermilk
1/3 cup vegetable oil
3 eggs
1 tablespoon unsweetened baking cocoa
2 teaspoons royal blue paste food color
1 toothpickful violet paste food color
Frosting
1 jar (7 oz) marshmallow creme
1 cup butter or margarine, softened
2 1/2 cups powdered sugar
1/8 teaspoon salt

1 box white cake mix 1 1/4 cups buttermilk 1/3 cup vegetable oil 3 eggs 1 tablespoon unsweetened baking cocoa 2 teaspoons royal blue paste food color 1 toothpickful violet paste food color 1 jar (7 oz) marshmallow creme 1 cup butter or margarine, softened 2 1/2 cups powdered sugar 1/8 teaspoon salt

Heat oven to 325°F. Grease and lightly flour bottoms and sides of three 8-inch round cake pans, or spray with baking spray with flour.
In large bowl, beat cake ingredients with electric mixer on low speed about 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally.
Divide batter evenly among pans. Bake 22 to 27 minutes or until top springs back when lightly touched in center. Cool 15 minutes. Remove from pans; cool completely.
Spoon marshmallow creme into large microwavable bowl; microwave uncovered on High 15 to 20 seconds to soften. Add butter. Beat with electric mixer on medium speed until smooth. Beat in powdered sugar and salt until smooth.
If necessary, trim rounded tops of 2 cake layers to flatten before assembling. Place 1 cake layer, top side down, on serving plate; spread with about 1/3 cup frosting. Top with second layer, top side down; spread with about 1/3 cup frosting. Top with untrimmed cake layer, top side up. Frost side and top of cake with remaining frosting. Store loosely covered.