So I'm hoping someone can clear up my confusion on this.  I'm doing a wedding cake for a friend and because of budget reasons she has decided to do a small two tier presentation cake and then sheet cakes to serve to the guests.  The presentation cake is going to be covered in purple fondant with some white scroll work.  Should the sheet cakes also be covered in purple fondant or is it okay to cover them with white or purple frosting?

Hopefully someone can help......

You will need to ask your friend in case it matters.

I thought there might be some proper etiquette when it comes to this or common practice.......

The idea behind the kitchen cakes or sheet cakes is to have more cake available, but cost a little less since it is easier to do.

Wedding cake slices rarely have all of the outside decorations on them.  Many slices are cut from the interior sections of the cake. 

With that in mind, a kitchen cake has all of the layers and filling of the presentation cake, but only have a simple frosted exterior.  This is often a way to cut cost, but still provide good cake to guests.

I would NOT decorate kitchen cakes for that exact reason.

I inquire with the client when I have an order for a kitchen cake. I have had a couple want coordinating colors and icings, and a few just want the cheapest option ( white buttercream ) regardless of the wedding cakes colors or icing type.