have you ever done it? how do you make it not show through?? I have a request to do a chocolate cake with chocolate icing but it would force me to put white fondant on top of it to decorate with.
I did that once and it was fine, you just have to be really careful and make sure you roll out your fondant thick enough so it can't possible show through.
I do it all the time--all sorts of frostings that are other than white underneath... if you can see the chocolate icing through the fondant, then you're just rolling your fondant too thin.