I'm making a rainbow cake for my daughter's 4th birthday. The inside is a surprise for her but the outside is going to be fondant flowers, grass, butterflies and a rainbow on sky blue buttercream. The cake itself is just a plain vanilla sponge and although it tastes OK it's a bit boring.

What flavour should I make the buttercream so that it tastes as good as it looks?

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.  

I like the idea of the white chocolate buttercream. I have made this recipe several times and it is AMAZING! I could just eat it out of the bowl! haha!

http://comfortablydomestic.com/2011/06/28/white-chocolate-buttercream/

I made on last weekend and I used SMBC, but I colored the frosting to match each cake layer color so when I used it as filling and it came out great. Woodland bakery has a great tutorial on this.