Chlorophyll is a molecule containing many atoms which all have different "excitation states" available to emit (reflect) light. Thus, all light is absorbed and many wavelenghts (mainly in the green spectrum) are emitted.
One molecule of chlorophyll is like every other and has the same transition state.
Actually, there oare two types of chlorophyll molecules but that is beside the point. There are many transistion states with those with energies corresponding to the greens emitting while the other frequencies of light are not (at least for the most part) emitted but the absorbed energy (exitation) is converted into molecular re-actions.
The only change would be slight variations in its chemical environent that can casue the energy for the transition to shift slightly higher or lower, hence casuing a broadening of the wavelength window responsible for the observed color.
Again, it is a molecule and has many transition states resulting in many frequencies being emitted (reflected). You are partially correct in that bringing atoms and/or molecules in close proximity results in additional transition states (frequencies).
Chlorophyll is a poor example because of its complexity and the multiple transitions that occur in the visible. My point was that each of these transitions involves changes between specific quantum states and that a continuum of states does not exist for a specific transition in a molecule. Chlorophyll, as you mentioned, has multiple transitions, but these each have a different, specific origin. Only a few of the atoms in chlorophyll are directly involved in transitions in the visible, and some arise from the conjugated system around the central iron atom. All things being constant, the same molecule will have the same transition energy involved with aborption for a specific transition.
Also, using color is also poor example because of the complexities involved in the biological perception of color. I say this because in vision, the observer is as critical as the source and the object observed. Light observation also involves quantum transitions in the photosensitive chemicals in the photoreceptors of the retina. The same apparent color can be produced by the proper balance of stimulation of the three tristimulus functions of the eye, even though completely different wavelengths of light may be involved. That's an interesting phenomenon in cheap fluorescent lights. They are really a mix of a red and a green emission bands that are balanced in such a way that they look bright white to thenhuman eye.