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