Why does the colour of a phosphor's glow depend more on its energy levels than on the energy of the exciting particles?

The color of a phosphor's glow is determined by the specific electronic energy levels of the crystal centers, rather than the energy of the exciting particles, because of how luminescence is physically processed within the material.1

When a phosphor is excited—whether by photons, electrons, or other particles—the excitation energy is used to lift electrons from their ground state to a higher energy level. In many cases, the energy provided by the exciting source is significantly higher than the minimum required to reach these radiative levels.1

Once excited, the electrons undergo rapid internal processes, such as relaxation within the crystal lattice, where excess energy is dissipated as heat. The electron then returns to its ground state from a specific, characteristic excited-state energy level, emitting a photon in the process. Because the emission occurs during this final transition between defined energy states of the phosphor's centers (or activator ions), the wavelength—and thus the color—of the emitted light is dictated by the energy difference between those specific levels, not by the initial, often excessive, energy of the exciting particle.12