Clouds condense rapidly on one side of the mountain while the other side is clear. while standing on top of the mountain and being close to the area of condensation an intense light as the sun behind you will allow your Shadow to cast upon the condensation.
The spectrum of light that passes through the condensation is refracted into a prism of color just beyond your shadow allowing a rainbow effect as when the sun sets behind other mountains or extremely dense dark clouds allowing rainbows to happen.