A fog bow is comparable to a rainbow, but because of the very small size of water droplets that cause fog, smaller than 0.05 mm, the fog bow has no colors and appears white. Fogbows are sometimes called "white rainbows" or "cloudbows". Mariners sometimes call them "sea-dogs".
The fogbow's relative lack of colors are caused by the relatively smaller water drops... so small that the quantum mechanical wavelength of light becomes significant and smears out colors that would be created by larger rainbow water drops.
This is significantly different from a moonbow, where the colours exist but are regularly not seen because the moonbow is so faint.