Fs is the frequency at which whatever your measuring is most efficient at vibrating

So it’s a combination of the composition of the thing and the environmental coupling with other vibrating things

Size and material composition are the primary factors

So for this case, the photon spectrum interact with nitrogen-oxygen mixture most efficiently at the frequency that reflects blue

I mostly studied sound frequency mixing with static objects (matching or cancelling the fs of room/space with the fs of a driver) but the principles of resonance hold across media