Icing and food coloring are a match made in heaven, but for unusual frosting colors, like tan, the coloring process is a bit more challenging. While tan food coloring is available, it's often much more difficult to find than the standard packages of primary and common colors. Luckily, you can mix your own tan food coloring shade and use this to color your icing exactly as you want it.
Mix some brown food coloring by combining all three primary colors--red, blue and yellow--since this creates a mixture of two colors opposite one another on the color wheel (this is how brown is made). Combine the colors in a small dish and stir with a toothpick. Start with a ratio of three parts yellow, two parts red and one part blue.
Test the food coloring in an opaque white glass filled with water. This will give you an accurate impression of what your food coloring mixture will look like when mixed with white frosting. Remember that once you like the color itself, you can make it darker or lighter by controlling how much food coloring you add to the frosting.
Adjust the color of your food coloring mix. In order to get the shade of color you want, add more of certain colors to your mix to get the right balance. Try to determine which primary colors you need more of. You may find that the right balance comes from adding one or two colors at a time; for example, if the mixture is too red, you may need more of both blue and yellow, or you may just need more yellow if you want your tan to be more orange than pure brown, so always experiment with one color and small amounts at a time. Test each new color mixture with a fresh container of water.
Add food coloring to uncolored, white frosting. Mix your white frosting well, then add the finished brown food coloring. Add just a drop at a time and mix fully. Then decide if you want your tan to be darker and add a little more. The smaller the amount you add each time, the less likely you will be to accidentally make the frosting too dark a shade of brown.