I believe except for a few pieces of exterior plastic (for example the fender skirts, mohican trim and gas door) the insight is an aluminum vehicle, the unit body as well as the exterior body panels are aluminum. Also the primer is special but the top coat of paint is the same as any other automotive paint.
The odd colors are a combination of how many different colors the factory is willing to use and the choices of the stylist. Having worked with a Japanese company before the stylist is probably picking colors he thinks will appeal to westerners.