It's a red bow? It isn't raining? The fact that the sun had set, where is it getting it's light source from?
OK, here's my guess.
It doesn't need to be raining
where you are to see a rainbow. The light from the Sun hits the drops of rain falling and is refracted back to your eye. So you can see a rainbow after the sun has set, because the sun is still shining on the rain falling from the cloud. The Sun can be seen shining on the clouds behind the rainbow; and the rainbow does not touch the ground because the light from the lower part of the rain curtain isn't illuminated.
The light is mostly red because of the scattering of the shorter wavelengths as the light travels a long distance through the atmosphere. Which is why you see the disc of the Sun red at sunset, or indeed why Jupiter looks yellowish or orange (depending on how hazy the air is) this year.
The bow isn't pure red, the other colours are there but are faint. There is a trace of the outer bow as well.
Interesting and unusual, all the same.
