My second try with royal icing came out a zillion times better! It dried up hard and shiny in about an hour, unlike that goopy mess from the first batch. The keys to my success: a new recipe and gel food coloring.
The liquid food coloring just thins out the icing so much that it takes forever to dry. And when it did dry, the icing became separated and grainy. These gels worked out great, along with the squeeze bottle with piping tip.
I let the base coats dry before piping decorations on top.
The green set.
This time, I went to Baking911's page on Royal Icing and used the recipe for Outline Consistency. Here's how I adapted it:
2½ tbs meringue powder (in the baking aisle, or at party or craft stores)
8 oz. confectioners sugar
¼ cup lukewarm water
Mix for 10 minutes on high speed with the paddle attachment. Spoon out a third of the mixture into an airtight container -- this will be your portion for piping. Turn the mixer back on and add 2 tbs water. Scrape that into other airtight containers and stir in colorings. If it's too stiff for your needs, add a tiny amount of water; if it's too runny, add confectioners sugar until it gets to the right consistency.