Cloud Bow ~ Imaged by Kiran Chakravarti (photos) in Hertfordshire, England.

".. one of two seen today.  There was a deck of cumulus cloud onto which this cloud bow was projected.  It lasted for about 15 minutes and at first it was colourless, similar to a fogbow but then I was surprised to see some strong colour towards the red end of the spectrum.  You can also see some faint blues and greens."   ©Kiran Chakravarti, shown with permission
Cloud bows are rare and perhaps more so because they are easily overlooked. They can appear in low contrast against grey cloud and even blue skies.

They are formed by small water droplets suspended in damp air or sometimes by virga, fine rain that evaporates before reaching the ground.

Depending on the droplet size they can range from almost white and broad to narrower with increasing rainbow colours. The larger the droplets the closer in appearance to a rainbow.
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Here are Airy simulations of fogbows/cloudbows/rainbows from monosized droplets of increasing diameter. Diameters are in microns.

Diffraction considerably broadens the bows from small droplets. The colours overlap strongly to give almost white. Oddly, the inner supernumerary fringes have more colour. These are also washed out when there is a range of droplets sizes.