"The cipher once solved will narrow that down for you. Then you can go to the places in the poem or search them from your PC. Whatever you prefer. "
That's the exact comment from Dolos. For me, it tells me that one of the cities is the correct city to start.
btw, as per your last post on rainbows - there is a famous spot in the Great Smoky Mountaains called Rainbow Falls :) a coincidence? maybe...