If you love character bentos (kyaraben), you may wonder how the creators manage to color some of the elements. Do they use food dyes? Not necessarily. Bentobako.net, called Ranchi-ryuu Obentoubako Community (Lunch-style Bento Community) is an attractive and very useful bento resource site in Japanese. One of the most interesting sections that they have recently set up is the Bento Coloring Dictionary, a reader-contributed section with lots of ideas for making various bento elements colorful without resorting solely to food dyes. Here are some of the ideas listed there, arranged by color, with my notes in parentheses.
I've added the formulas for deriving 3 (or 4) different colors from one ingredient - red cabbage juice!