When you cover a cake with colored buttercream everyone worries about how the guests will end up with dyed tongues. Not only that, but a lot of certain food colors will make the icing bitter or just "off" tasting.
This isn't a difficult concept so I won't do a step-by-step tutorial...just cover the cake in white, then when you do the final thin coat of icing use the colored icing. The only thing you need to watch out for is making the colored coat TOO thin, since that might pick up some of the white icing in it when you smooth it out. If you refrigerate the cake after the first coat of white icing until it hardens up, it will be easier to cover the cake with the colored icing without smearing the white.
Kara Buntin owns A Cake To Remember LLC, online cake supplies at www.acaketoremember.com and www.acaketoremember.etsy.com