Whenever I try to get a dark red for Christmas cookie frosting, I have to dump about a tablespoon per 1 & 1/2 cups of frosting. Then the frosting tastes bitter. 
One trick I learned was to add some cocoa powder. It makes the frosting a little darker, but also a little less vibrant. But then again, I am only using the standard food coloring.
Do you have any tricks or tips to share?
