For my 5 year olds birthday I'm making a chocolate sheet cake, and I'm going to fill it with a peanut butter mousse, Not sure that I want to do peanut butter frosting though as It seems like it wouldn't be easy to color, and I'm going to want different colors, would a regular buttercream work in combo with the peanut butter filling?
Welcome to CC! I would use a vanilla buttercream. It works well with chocolate cake and pb filling. And you'll have no problems adding color.
I'd enjoy a nice vanilla buttercream on top also, maybe with some Reese's Peanut Butter cups.
here are a few previous conversations: http://cakecentral.com/t/758987/peanut-butter-buttercream