An excited atom decays to its ground state and emits a photon of green light. If instead, the atoms decays to an intermediate state, then the light emitted could be:

When an atom decays from a higher energy level, the photon energy equals the energy difference between levels. Green light has higher energy than red, so if an atom emits green when decaying to the ground state, a smaller drop to an intermediate state would emit lower energy light like red, which has a longer wavelength.