The brightness of the different colors depends also from the composition of the light. When the sun is low above the horizon there is relatively more red in it likewise you can see in Rainbow 1 and Rainbow 2, and when the sun is high there's relatively more blue in it. And it depends of the state of the atmosphere. The little bands at the inner side of the rainbow (violet and beyond) are difficult to be explained (but perfectly showed in this shot). Even now there are riddles that are not entirely solved....