There are three categories of Bread Monster Families, White Bread, Brown Bread, and Dark Bread. They will keep their own colors until 12 years old. Since 12 years old, they can see pigment molecules floating in the air, and color their body, change their appearance with pigment molecules.
Rainbow Bread is from a Monster family of White Bread and was born as other White Breads. His skin had no color but pale white so loves color so much. We can even say he is mentally affected by colors. His wish is to collect all kinds of different colors all over the world. On a day of his 3 years old and he saw a colorful rainbow hanging in the sky. He was so amazed by the rainbow. Since then he wishes that he could turn to 12 years old imminently and color himself like rainbow.