Make cake following package directions and put into a jelly roll pan and bake as directed. Cut two rounds to fit trifle bowl.
In a glass Trifle dish, place one of the cake rounds into the bottom. Top with prepared instant chocolate pudding mix. Place banana slices along the sides, not too close together.
Add another layer of the yellow cake. Scatter blueberries or blackberries along the top and sides of this cake layer.
Top this layer with prepared vanilla pudding. Place strawberry halves along the sides, not too close together.
Top the entire top of this trifle with Cool Whip and sprinkle various Christmas sprinkles, including red and green trees to make it look festive.
Note: I used leftover whipped cream and it did not look as presentable as when covered with Cool Whip.