Combine the powdered sugar, milk, coconut oil, and vanilla in a large bowl and whisk until uniform. If the frosting is looking too thin, add a little more powdered sugar. Divide the icing among 4 bowls.

Tint the icings with food coloring until the desired colors are achieved. You'll need only a few drops, as gel coloring is strong.

One color at a time, recombine the icings back into the first bowl, or into a pie pan. Une a spatula to swirl the colors together just once. Do not over mix or you will get a muddy color.

Dip the doughnuts into the swirled icing, twisting slightly to evenly coat the tops. Remove from the icing and place on the wire rack set above the cookie sheet to rest. Sprinkle with edible glitter, if using, and let sit until the icing hardens before enjoying.