Bake cake according to package directions. While the cake isbaking, pour half the sweetened condensed milk (1/2 cup), frozen blueberries and blue food coloring, if using, into a blender and blend until smooth. Pour into a squeeze bottle or a jar with a narrow opening. Rinse the blender and pour the remaining sweetened condensed milk, strawberries and red food coloring, if using, in the blender and blend until smooth. Pour into another squeeze bottle or jar. Set aside while cake finishes baking.
Allowvcake to cool for 5 minutes then poke holes using a skewer or the end of avwooden spoon, in rows.
Pour the prepared berry mixtures in rows (widthwise), alternating to make a red and blue pattern. I made each row about an inch wide. You could go narrower or wider depending on the look you wish to achieve. Cover with plastic wrap and place in the refrigerator to cool completely.
Meanwhile, make the frosting. Mix together the pudding mix and milk until thickened. Gently fold in the Cool Whip. Spread over the cooled cake. Decorate with blueberries, strawberries and sprinkles.