My rainbow cakes are becoming pretty popular for my business, and I typically use my vanilla buttercream between the layers. I do use a variety of vanilla flavors in my frosting, including a high quality vanilla extract, so the vanilla flavor is anything but boring. However, if you want a flavor other than vanilla that keeps the frosting white, you could do white chocolate, or add extracts or emulsions that don't change the color a lot, like adding orange emulsion to vanilla buttercream for a dreamsicle flavor, or rasberry extract, or lemon extract, etc etc. Now that I think about it, I've sometimes included pink as part of the rainbow, so you could also do something like strawberry or rasberry flavored buttercream. I tend to stay away from fruit fillings for the rainbow cakes because in my opinion having that strong color filling as you cut detracts from the rainbow effect.