Seeing a moonbow
Seeing a moonbow, a rainbow made by moonlight, takes a bright moon, falling water and little other light. To the eye it looks white or silver, since moonlight is too dim for us to see its colors, though cameras pick them up.
What it may mean
A moonbow has every color a rainbow has. You just cannot see them. At night the eye relies on cells that sense brightness but not color, so the arch over a waterfall on a moonlit night looks like a pale ghost of a rainbow, white or faintly silver. Its red and violet are really there, as a long camera exposure will show. Many things in life sit below the threshold like that. A quiet colleague's humor, a teenager's affection, a friendship that seems gray only because the two of you meet when you are tired: the color is there, but the light is too low to show it. A moonbow suggests changing the light before judging the color, by meeting someone at a better hour, or in a brighter mood, and seeing what was there all along.
Depending on how you saw it
At a waterfall when the moon is nearly round
Moonbows are most reliable at big waterfalls whose spray hangs in the air, such as Cumberland Falls in Kentucky, Victoria Falls and Yosemite, on cloudless nights close to full moon. Which sight would you arrange a whole trip around just to see once?
After a late shower
Sometimes a moonbow forms in ordinary rain, when a shower passes on one side of the sky and a bright moon hangs low on the other. Few people see it, because few are outside to look. What would you see if you stepped out after the rain instead of closing the curtains?
Why it happens in nature
A moonbow forms the same way as a rainbow, by light refracted and reflected in water drops, but the light comes from the moon. It needs a bright moon near full, low in the sky, with rain or spray on the opposite side. Moonlight is so faint that human color vision barely works, so most moonbows look white, while long-exposure photographs reveal their colors.
To reflect on
Whose colors might you see if you met them in better light?