• Use gel food coloring: using powdered or watery food coloring can affect the consistency of your royal icing. Gel food coloring is extremely pigmented so you shouldn't need much, and the consistency is already like that of the icing itself.
• Royal icing darkens as it dries: whatever color you want on your cookie, you'll want to mix it slightly lighter as it will darken as it dries.
• Some colors are hard to mix: red and black in particular can be difficult colors to mix. The good news is that they will darken as they dry. It also helps to use a highly pigmented gel food coloring like "super black" or "super red" by Americolor.
• Make more of each color than you think you'll need: because royal icing darkens as it dries, it's very hard to match a color if you end up not having enough. It's always a good idea to make more of a color than you think you'll need.
• If you need an outline for your design, draw it directly onto your cookies with a food pen/marker.
• For easy loading, place the piping bag into a tall glass and pull the top down around the sides of the glass. This will hold the bag in place while you pour.
• Use a toothpick or a quilling tool to work the icing into place. A gentle tap or two will help the icing settle.
• If you end up with any little air bubbles, you can pop those with your toothpick or quilling tool.
• Allow the icing to dry completely before packing or adding additional decorations (about 6-8 hours).
• To prevent butter bleed, cool your cookies on a cooling rack rather than the pan, make sure they're completely cool before working with them (I like to leave them out over night so they cool and dry as much as possible) and use a thicker icing (a thinner, more porous icing is more likely to suck up more butter).

Adapted from Bake at 350