Are there any poetic names for a "rainbow"
Solution 1:
showery prism,triumphal arch,lovely Iris are all candidate phrases that may be found in the poems of James Thomson and Thomas Campbell.
https://interestingliterature.com/2019/10/9-of-the-best-poems-about-rainbows/
https://discoverpoetry.com/poems/rainbow-poems/
I like the showery prism because of its physical accuracy and its suggestion of spectral colours.