When you add colors that oppose each other on the color wheel (that would be complementary colors) you get a grey/browned down version of the original colors. I added a couple of drops of spring green to the pink icing to grey it down, and it was enough to give it a pastel feeling as opposed to being so pink.
The one on the left was the original, and the one on the right is the greyed- down version. The difference is subtle, but it will look different on the cake.
The complementary colors are: red--Green
Adding those colors will give you a toned down version of whichever you have more of. Just add the complementary color bit by bit and go slow. You can always make it darker, but you can't make it lighter. Too much of both colors will give you a grey mess.
Kara Buntin owns A Cake To Remember LLC, online cake supplies at www.acaketoremember.com and www.acaketoremember.etsy.com