For the frosting, we pained on watered-down royal icing in white. Then, we piped on runny royal icing in green and blue. They turned out awesome!
Just for the record, we used two types of brushes to paint the base coat on the cookies. One was a boar bristle 1" basting brush made by OXO. The other was a silicone basting brush that we have had lying around in the drawer. I went to Michael's again and I did see some small brushes made by Wilton, but I had already talked to the experts at the local kitchen shop, who said a basting brush would work, and they were right!
-吴碧芙