Rainbows can normally only be seen if the sun is low, as otherwise you have to look down to see them. At sunset, a rainbow can form a semicircle. This picture was taken from my house in the late afternoon, when the primary rainbow is actually below the horizon. Note that the secondary one has the colours reversed.
This picture is actually a montage of two photographs, as my camera lens couldn't zoom out far enough to fit everything in.
In contrast, the photograph on the right was taken as the sun was about to set. Notice how the rainbow almost meets the horizon at right angles. The red sky seems to have caused the rainbow to be predominantly red.
I have occasionally seen rainbows which appear to have extra colours beyond the purple - additional green and violet arcs. This is probably some distortion effect causing a second overlapping image, not extra colours. This can be seen in the picture below, with the second photograph showing a detail with the colours emphasised.
