Those perfectly iced sugar cookies you see particularly around holidays use royal icing, which is thinner than frosting and hardens with a glossy sheen. Royal icing is made with confectioner's sugar, egg whites and lemon juice. Plain royal icing is white, but you can use food coloring to make any color of the rainbow. Color gels make major color changes with just a small drop and won't thin out icing like watery food coloring. Color gels come in all colors, including brown, but you can make brown from primary colors without making a special trip to the store.
Insert a toothpick in the red color gel. Hold the toothpick over the royal icing and allow 2 drops to fall into the icing. Repeat this with 2 drops of yellow color gel. Add 1 drop of blue color gel.
Stir the royal icing well, using a spoon or small whisk to thoroughly incorporate the three colors so you have no streaks in the icing.
Add more of the red, blue and yellow food gels at a ratio of 2 drops of red and yellow to 1 drop of blue to make the brown darker. If you have a large batch of icing, you might be able to add more of the colors at once to make major changes quickly, but single applications are best to make the shade slightly dark. Mix the icing thoroughly each time after applying the three color gels so you can determine if you really need to add more color. Repeat this process until you have the shade you want.