Answer
Red photons have less energy than blue photons. Blue light can expose the film. It would be safer to have bright red light than very dim blue light.
Work Step by Step
Red light has insufficient energy per photon to trigger the chemical reaction in the film. Bright red light just means more photons, each of which can't cause a reaction, so it is still safe. Blue light has enough energy per photon to expose the film, so the less blue light, the better.
