Prepare the yellow cake mix following the directions provided on the box. Pour the dry mix into the bowl and add any necessary ingredients. Blend thoroughly using the mixer.
Heat the oven to the temperature indicated in the package directions.
Spray the oval baking pan with non-stick baking spray and pour the prepared cake mix into the pan. Bake according to the package directions.
Remove the cake from the oven and place it on a wire rack to cool to room temperature, at least 10 minutes.
Turn the cake pan over gently and let the cake drop onto the wire rack. Transfer it to a serving platter.
Spread the white frosting over the entire cake with the spatula.
Fill a decorating bag with pastel frosting. Outline the top edge of the cake with this color.
Fill another decorating bag with a second shade of frosting. Pipe curved lines, dots or rosettes from one side of the egg shape to the other to simulate bands of egg dye. Repeat this process with as many frosting colors as you like.
Press jelly beans lightly onto the cake to add further adornment. The cake should now resemble an Easter egg.