in addition, try concentrated beet juice (reduce the cooking liquid from boiled beets) and blueberry juice (reduce the juice that runs off from defrosting frozen berries).

Use turmeric for yellow, the juice of frozen raspberries or pomegranate for red/pink, cocoa for brown, saffron for a crimson-y yellow. Those are the ones I know will work and they won't add too much flavor to your icing, used in small quantities. Also, some liqueurs will lend color (raspberry di amore, etc.).

Here's a link that has some ideas:
http://lifestyle.iloveindia.com/lounge/how-to-make-food-coloring-4219.html