Green photons are more energetic than red photons.
Work Step by Step
For photons, E = hf, where E is the energy, h is Planck's constant, and f is the photon's vibrational frequency. A higher-frequency green photon has more energy than a red photon. This is discussed on page 564. The incident photons from the photoflash tube must have at least as much energy as the photons they are stimulate in the laser. Red photons have less energy than green photons, so red illumination from the photoflash tube wouldn’t be energetic enough to stimulate the green crystal rod to emit green photons.