IANAP, but I don't think that photon energies are quantized. That is, a photon can have any energy. But the sizes of those energy/momentum packets are quantized

IANAP, but I don't think that photon energies are quantized. That is, a photon can have any energy. But the sizes of those energy/momentum packets are quantized. But energy levels in atoms are quantized, and each sort of atom has a distinct set of possible energy levels. And so can emit/absorb a distinct set of possible energy photons. That's what emission/absorption spectra are all about. Free-electron lasers, conversely, can be built to produce coherent photons of any energy. Albeit constrained by construction methods.