The Rainbow Reminds Us of Enoch
The rainbow is to remind us of a promise first made to Enoch:
21 And the bow shall be in the cloud; and I will look upon it, that I may remember the everlasting covenant, which I made unto thy father Enoch; that, when men should keep all my commandments, Zion should again come on the earth, the city of Enoch which I have caught up unto myself. (JST Genesis 9:21)