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Beat the confectioners' sugar, butter, milk, and vanilla until the mixture is smooth and creamy. Add more sugar if it's too runny, or more milk if it's too stiff. Put equal amounts of frosting into each bowl. Squirt a few drops of red food coloring into the first bowl, blue into the second, green into the third, and yellow into the fourth. Stir each bowl of frosting and add more coloring until you're happy with the colors.  If you want to make unique colors, mix two types of food coloring in one bowl of frosting. You could mix yellow and red to make orange, for example. Gel food coloring tends to create brighter colors than liquid food coloring.

Summary:
Beat the icing ingredients in a medium mixing bowl. Divide the frosting into four bowls. Add food coloring to the bowls.