use red red or super red coloring gel from americolor. wilton also has a good selection for red coloring gels too... forgot to metion, wilton makes a christmas red which is a nice deep red, hope this helps. 
I just made red mmf, and black buttercream but haven't tried red buttercream yet, i have heard if you start with pink buttercream then add your red it may work.
for my mmf red i used a powder color red, practically the whole bottle, which isn' a good thing when it costs $15.00 a jar....lol
someone else may have some other answers for you so good luck and post a pic when your done!
kylie
I'm using ChefMaster liquid gel, so far it worked for me although I have to use quite a bit to get the effect that I want.
One trick I've learned is to start coloring a few days before you will need it. The longer it sits the darker it gets. The same is true with black and blue.
I only have Wilton colors so what I do is use "No Taste Red" for the majority of my coloring. Then I give it a boost by adding a small amount of either "Christmas Red" or "Red Red". Those two colors tend to have a bitter taste so that's why I only use them in small amounts.
Also, the red will darken more once it's applied to the cake.
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