It was a double rainbow that arced unbroken across the sky. I guess that means there were two crocks of gold at each end.
The space between the bows looks distinct from that either side of them. This is how you get ancient peoples believing there's a rainbow bridging our world and another.
OK, so this rainbow was intense, but it wasn't on fire. Still, I'm sure that's a minor detail that can be worked around for those who want to pass between Midgard and Asgard.