How to process video with DFT/FFT - getting the frequency of green color changes?
If you need to track changes in time, then the most natural and obvious way is to form one temporal (green data) vector per pixel that you want to observe, then take its DFT (it's way 2, but with more than 2 frames). 

The actual vector length and DFT function that you should use will depend on the actual case you are working on. 


Suppose you work from a recorder video. Then, you can take the 1D DFT of the vector formed with the value of one pixel of interest in the whole sequence.
On the other hand, if you work from a live feed, then you need to use a FIFO data vector and probably apply a windowed 1D Fourier transform to avoid artifacts, see short-time Fourier transform.