While the usual use for food coloring is to shade frosting, you can also use food coloring to color cookie dough, play dough and a variety of other foods. Tan is a difficult color to achieve with food coloring because it's neither a primary nor a secondary color. Tan is basically light brown -- a tertiary color made by combining red and green or orange and blue. The simplest way to make tan food coloring is by starting with a professional food color shaded brown and toning it down with yellow.
Professional Food Coloring
Professional cake decorators use paste or gel food coloring. These products come in a range of vivid hues, so you need just a tiny bit. To make tan frosting or food coloring with paste food coloring, combine 5 parts yellow and 3 parts brown. If you're using gel colors, start with a tiny bit of brown. Added to buttercream frosting, the brown will appear tan.
Liquid Food Coloring
You don't have to buy professional cake decorating colors, especially if you rarely use food coloring. You can make tan from bottles of liquid food coloring from the grocery store. To make tan, combine 1 drop of red color with 10 drops of yellow. If the color isn't brown enough, add 1 drop blue. Liquid food coloring doesn't give quite the same versatility as professional food coloring, but in a pinch, it will work.
Whichever type of food coloring you use, start slowly. You can always add more if the color isn't dark enough. When mixing frosting, such as buttercream frosting, always beat the color in thoroughly before adding more. Keep in mind that acidic ingredients, such as cream of tartar and lemon juice, can alter the color of the frosting. Minerals in tap water can also cause changes.
Wait a Minute
Frosting shaded with food coloring becomes darker when you store it. Make the frosting so the shade of tan is slightly lighter than what you want. Refrigerate the frosting for at least two hours and come back to it. If it still looks too light, add a bit more color. It will probably have darkened to exactly the right shade. If the frosting is too dark, add a bit more plain frosting to lighten it.