Not stupid. The answer isn't in the printed rule book that came with the game, but can be found in the electronic rules found at the Cranio Creations web site.
Electronic rules, page 6 wrote:
White bonus dice
A white bonus die can be used as a die of any color (players
can decide after the throw which color it should represent).
However, the white dice are one-time use only. But they are
the heroes' only hope when they fail to kill the monster with
the three hero dice.