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.

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The best way to avoid a colored-icing overload is to cover the cake in a regular coat of white icing, then apply a thin layer of the colored icing over it.
The colored icing will be minimized but it will still provide the color on the cake, and your guests won’t have to deal with as much food coloring in their mouths.
You can also make black details out of fondant instead of piping them on. This takes more time, but it will avoid having to deal with black buttercream, which can be really problematic. I wrote an article about that here.
This cake is a good example of this, since I used a burgundy icing for the top section of the tiers and white underneath it. The black parts are fondant cutouts that I applied to the cake after the burgundy icing was added. They covered up any rough edges where the dark icing ended and the white color started.
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.
Don’t make the icing too thin.
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.
Return the cake to the fridge if the white icing starts to soften up, keep the surface hard while you’re adding the darker icing to it.
You can also use a piece of waxed paper or parchment to cover the white icing while you’re applying the darker buttercream to act as a mask for the white.
When I did this cake I used waxed paper to cover the white sections, iced the burgundy on, then added the black fondant sections to create the archways and hide the area where the colors touched.
That way I could easily cover up any uneven edges and make the design even all the way around.