Add more red food dye. Make sure you are using gel paste colors NOT the liquid colors sold in the grocery store. Also, Americolor is better than Wilton, you get brighter, darker hues with less of the food dye than with the Wilton. However, you can acheive the darker colors using the Wilton gels. Adding black will not deepen any color, it will just make it muddy. Also, colors deepen with time. Color your frosting and let it sit. After an hour or two, it will "ripen" to a darker color!
I add 4oz of cream cheese to my buttercream when I need to make red and never have a problem. The longer is sits, the more deep the color gets.
For red and black that's dark with not a bad taste I use only Americolor gels. They are worth ordering. I add some pink first then the red and let it sit I never use the red or black the day I make it up unless it's an emergency. As for black I take all my unused colored buttercream that is leftover and put them in a large bucket and add black each time I add a color and let it sit. You get deep vibrant colors by waiting overnight.
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