In a thunderstorm cloud about sunset time, I saw these clouds, including some (in the upper right) that were a unique shade of blue. I don't think I've seen clouds quite that color before. I tried to capture the color as best I could with a digital camera, though the blue is slightly washed out in the image. I took these images facing East at sunset time, during summer.
Optically zooming in on the blue cloud:
This answer suggests that the sunlight should be missing those wavelengths ("at sunset, when only the red/yellow end of the spectrum hits") which is consistent with the brilliant warm colors to the left. But...the cloud was a strong and unique shade of blue, different from the blue behind it. How does this happen? That is, what natural processes underly the occurrence?