I'm assuming you could use RI. I use the following:
3 cups powdered sugar (sifted)
3 Tablespoons skim milk (you can use water or other type milk)
3 Tablespoons corn syrup
15 drops brite white food color, optional (I use Americolor brand)
Mix the milk and corn syrup into the powdered sugar until well combined (I use a bowl and a spoon). Add the brite white and mix well. The brite white helps prevent the icing from getting cloudy and spotty, though I only had that problem when I refrigerated the decorated cookies. At this point, I play with the icing until I get the consistency I want, which is similar to white school glue. To thicken, add more powdered sugar. To thin, and more corn syrup. Divide and color.
For the final outlining icing, I add more powdered sugar until the icing is the consistency of peanut butter.
I use this recipe exclusively. I searched high and low for a good cookie icing recipe and I can't thank GeminiRJ enough for sharing her recipe. I do add almond extract to give it more flavor.