Take about 3/4 cup of royal icing into each of 2 bowls. Add water by the teaspoon to achieve a thin consistency. You want the icing to run off the back of the spoon and the drips to disappear back into the icing below within 5 to 10 seconds.
Once you have achieved the right consistency, add brown coloring gel to one of the bowls. This may take quite a bit of gel to get a nice, dark color. Then add the ivory food gel to the next bowl - this will take much less so add it slowly.
Once you are satisfied with your colors, add each of them to one of two plastic squeeze bottles. Use a small piece of plastic wrap to cover the opening if your bottles aren't equipped with a cap. The bottles are now ready to flood. Alternatively, use disposable pastry bags fitted with #5 tips for flooding.
After the outlining has dried for at least 1 hour, flood the sections as shown in the photo.
Use a toothpick to help guide the icing to the edges if necessary.
Let cookies dry for several hours or overnight. You can just let them sit out at room temperature, uncovered. You may also leave the black piping icing in the bag, just place a cap on the tip or cover it well with plastic wrap to prevent the icing from crusting over in the tip.