I make them all of the time. Put in the black until it get's dark gray. Let it sit overnight and it will turn black. with red, do the same thing. Put in the color until it's close and let it sit. Once you ice your cake, the colors will get more intense then also.
Just remember to start with a lighter base color and then add the darker color and let sit overnight. That's the basic concept here.
I add cocoa powder to my buttercream and then add black until it's close. Let it sit overnight and it will intensify. See this example that I made. Middle layer is primarily black.
http://www.cakecentral.com/cake-photo_1808231.html
For the red in the same cake, I added pink to the buttercream first and then added Wiltons red-red and let it sit overnight to intensify. This is what seems to work best for the dark colors. HTH!
This won't help right now, but for future reference, I save all my little bits of colored icing in a bowl in the freezer - just drop them in there when I have them. When I need black, I take that out and mix it all up, and then add enough black color to make it true black. Works like a charm!
Like the post Loucinda. How long does the buttercream last in the freezer?
believer- you will want to watch adding too much of the coloring, it could make the icing taste bitter. The trick with adding the color and letting it sit for a while does actual work not just with black and red but even with any deep hue of colors. Just be sure to plan ahead of time for it. Some people have posted buying the ready made. I personally don't like how it taste but that would have to be your choice. For black, if the individual does not mind have a little chocolate taste on the cake+no one is allergic to it, I mix cocoa into the buttercream then I add the black americolor.
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