Tint 1/2 cup vanilla frosting with yellow food coloring and tint 1/4 cup chocolate frosting with black food coloring; spoon each into a small resealable plastic bag. Place 1/4 cup vanilla frosting and 1/2 cup chocolate frosting into 2 bags; set all bags aside. Spoon leftover vanilla frosting into a 1-quart glass measuring cup. Microwave on high for about 20 seconds, stirring often, until the texture resembles slightly whipped cream. Place cakes on a wire rack over a cookie sheet, then pour frosting over beds to cover. Reheat excess frosting and repour if necessary. Refrigerate frosted beds until set, about 30 minutes.