You'll be hard-pressed to find gray food coloring, but you can make gray frosting by combining a few colors. Although buttercream frosting appears white, it's really more of an eggshell shade. Add a bit of bright white food coloring to the frosting first to get the purest shade of gray.
Paste food coloring, which comes in small lidded containers, is concentrated, inexpensive and widely available. To make gray from this type of coloring, insert a toothpick in a tub of black food coloring. Add just a bit of this color to white frosting and mix thoroughly. Add tiny amounts until you're happy with the shade. Frosting shaded with paste food coloring tends to darken as it sits. Make it a day ahead and stop when the gray is one or two shades lighter than you want it. You can always add a bit more coloring before you use the frosting if it still isn't dark enough.
Professional bakers often use gel colors because they're easy to use, last forever and they leave no metallic aftertaste -- a common problem with paste colors. To make gray using gel colors, add bright white coloring and a bit of black coloring to the frosting. Mix thoroughly and judge the color. If it still looks too light, add a few more drops of black coloring.
Liquid Food Coloring
Liquid food coloring -- sold at grocery stores -- is inexpensive and simple to use, but it comes in just a few colors. You may not be able to make a true gray with this type of food coloring, but for a close approximation, mix 5 drops of blue, 3 drops of red and 2 drops of yellow into 1 cup of frosting. Another option is to add equal amounts of red, blue and yellow food coloring to the frosting.
Tips for Success
Gray, which is a lighter version of black, is a difficult color to achieve. Too much color muddies it, while too little creates a wimpy shade. Real butter in buttercream frosting may also alter the color slightly. To increase your chances of success, substitute shortening or cream cheese for some of the butter in your frosting. Coloring isn't easily absorbed in Swiss buttercream because of its high fat content. Use more food coloring if you use this type of icing. Some shades of purple and gray can take on a blue hue, depending on the type of coloring you used. Add a bit of milk to buttercream frosting to help combat this problem. To fix frosting that's too dark, simply whip in more frosting.