Here's a rice ball bento box of the famous French characters Barbapapa which are still very popular in Japan. You need a variety of colored foods to mix with the rice. Here are some suggestions:
yellow- egg yolk, turmeric
green- green peas
purple- red shiso(an Asian herb used in Japanese cooking)
black- black sesame seeds
pink- sakura-denbu(a pink powder made from fish used in sushi dishes)
blue- This is the most difficult, you can use food coloring.
The face details are made with nori seaweed and cheese. The flowers are punched from carrots or other vegetables.