That is my other post converted in b&w. The fine structure of the rainbow is not visible anymore, all diffracted colours have the same intensity. The clearer sky zone inside the rainbow creates a natural framing for the landscape.
The idea of removing the colour from a rainbow image is very novel. And it has worked so well. As you said - it really accentuates that difference in saturation between the inside and outside of the rainbow. I have often wondered why that happens. :)
