Day of Rainbows

So many of us saw rainbows in the valley yesterday. This was the view from Doc's pasture just before noon. What intrigues me about this rainbow are the dark areas below the violet band on the two clouds. I think it has to do with the way the raindrops creating the bow are keeping the light from the cloud from getting back to us (backside refraction), but I haven’t quite worked out the geometry yet. I haven’t noticed this before, and I'm guessing the effect arises because the clouds were very close behind the source of the refraction causing the bow. Thoughts?