Prepare cake according to package directions, substituting half of the water called for with buttermilk (approximately 1/2 cup). Stir in pudding mix and food coloring.
Pour into cake pan(s) and bake according to package directions.
In a medium bowl, cream together the cream cheese and butter until creamy. Mix in the vanilla, then gradually stir in the confectioners' sugar. Store in the refrigerator for 10 minutes, then frost evenly onto cake.