Lots and lots of food coloring! Seriously, I usually use almost half a container of Christmas Red AND half a container of No Taste Red. Make it at least a day in advance and let sit (it will darken). It darkens more once you get it on your cake (not sure why).
Have you ever used Americolor gel colors before? They are the absolute BEST - much better than Wilton. Americlor Super Red (and Super Black) work great - you get a true red, not pink, and don't have to use very much so your icing is not bitter.
If there aren't any cake decorating supply stores near you that carry it, you can order it from many places online, including CC. 
Unfortunately I haven't personally had much more success with the Americolor compared to Wilton, I still use as much to get vivid colors. I was a little disappointed by that. Just BE SURE if you use Wilton, use NO TASTE RED. The other reds taste so nasty, it's not even funny! Just add a ton of it, then as mentioned, let it sit for a bit and it will darken. Also, the heat from your hand while you pipe it will darken it too.
Add the Kool-Aid (if you don't mind the flavor), as suggested in the pp, and a little cocoa powder, and in the name of all that is holy, use the no-taste stuff, or nobody will want it to begin with. Yick! Depending on the shade, adding a touch of other dark colors like brown (though I do prefer cocoa to adding brown) or blue will also speed it along and reduce the amount of coloring you'll have to add.
I've done the Kool-Aide twice. The first time I used a whole pack the Kool-Aide that has to have sugar added (the small pack) to a 1/4 batch of icing. It was sssoooo sour. So, I would suggest either using the kind with sugar already in it or maybe not using the whole pack.
funny - i just made my first RED buttercream and wondered why the tast changed....YUCK...i used wilton christmas red...now i will know to use NO TASTE or a different company...
so is americolor a good one? do you get icky tasting icing if you add too much colour?
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