You may have already seen my earlier posting on natural food colourings here.
• For blue, use red cabbage
• For red, try whole beets (not canned), cherries, or cranberries
• For light green, use spinach or fresh green herbs
• For tan, brew some strong coffee, tea, or a handful of cumin seeds
• For yellow, try turmeric (a spice) and yellow onion skins
• For olive green, use red onion skins (the color is produced by a reaction with the vinegar)
• For purple, grape juice or frozen blueberries.
Moving from American Easter eggs to Japanese lunchboxes, Maki of Just Bento has also written on natural ways to add colour to your food, based on suggestions from the bento-making community via Japanese-language websites such as Bentobako.net.
In summary, some of the foods used are:
Orange: salmon, carrot, paprika
Pink: ume vinegar, sakuraebi (small dried shrimp), red cabbage liquid with lemon juice
Purple: red cabbage liquid, purple potato powder
Blue: red cabbage juice with baking soda, purple potato powder, pickled eggplant liquid
Yellow: mashed kobocha (Japanese pumpkin), curry powder, turmeric, egg yolk
Green: mashed up edamame beans, peas broccoli or spinach
Brown: inarizushi skins, dark soya sauce
Black: nori seaweed
Grey: shirasuboshi (tiny little semi-dried fish)
And also how red cabbage prepared in different ways can produce purple, blue, red and brown colours! For full instructions, go to the original Just Bento post.