Deep colors require a significant amount of food coloring, which may adversely affect the flavor and quality of the icing. You will be asked to sign a dark color disclaimer when using these colors on your cake design.  These darker colors are available in marshmallow fondant without the possible adverse effects.  Sometimes splashes of these darker colors can be achieved by using a colorful cake board or stand, other sugar decorations, non-edible décor, or other means. I like to airbrush dark colors over a lighter colored buttercream to achieve the depth of color without the potential adverse flavor changes.