What's up with grey or colourless rainbows? Quirks question
The water droplets in a fogbow are not large enough to reflect and refract sunlight in the same way as raindrops do in a rainbow.
What appears to be a colourless rainbow is actually a phenomenon known as a fogbow.
Dr. Douw Steyn, Professor Emeritus in the Department of Earth, Ocean and Atmospheric Sciences at the University of British Columbia, explains that the ghostly bow differs from the colourful bow because of the size of the water droplets.
In the case of a rainbow, the droplets are large enough to act like a prism in the way they react to sunlight.
The water droplets that comprise a fogbow are too small to reflect and refract sunlight as raindrops can.