I'll bypass the garble in Genesis 6 of the numbering of human days, being 120 years, since I know without a doubt I'll never even come close to that age and I sincerely doubt anyone on this board ever has or ever will.
I'll skip all the nonsense of the Lord regretting his creating humans and all living things on the planet (after he declared them all good a few chapters before).
I'll avoid the endless arguments of trying to figure out how Noah got all species....sorry, kinds of animals,.... 2 by 2 or 7evens aboard a wooden structure, three storey ship. I'll even let it slip through that feeding all of these animals requires another miracle or three.
I'll skip over all the lacking of geological evidence of any worldwide flood.
I'll avoid getting into a discussion of Ken Ham preaching that Noah brought the dinosaurs on board (after all, he's not Mormon)
I'll dismiss the impossibility of genetic variation in present animals and humans developing in the mere 4000 years since the flood.
Let's just stick to what Genesis 9 says that God did in his covenant to Noah following the flood. (Genesis 9: 11-17).
Do Mormons honestly believe that, prior to the flood, that the optical division of light rays into a visible spectrum of six colours never happened? Now granted, the Bible doesn't mention anything about lawn water sprinkling systems and waterfalls are only mentioned in Psalms, but rain must have happened sometime between Adam and Noah. After all, when God tells Noah that he's going to cause rain for 40 days and 39 nights there is no excited expletives from Noah asking "What the Hell is rain?"
Wait a minute, Genesis 2:6 says there was a mist that watered the earth. Are we to believe that no one, ever, saw anything similar to a rainbow in this mist?
Think back to water sprinklers....mist.... When was the last time you looked at a mist? Did you see a rainbow?
I find it hard to believe that the physical properties of light dispersion were voided for the 1500+ years between Adam and Noah. Anyone here with an opinion?
a little later i will gladly dismantle your post's purported logic and confusions. (inadequacies at best)
But to answer your question
Does not a rainbow require light to pass through the "mist".
Is your assumption that this mist was occurring during a sunny and cloudless day?
Simply put, if we are to assume that a mist or rainfall covered the entire earth, then it is quite reasonable that the sun was blocked out by clouds.
Furthermore, are you not working on the assumption that rainbows had not been seen before?
The notion that they were created at the moment of the covenant is not something the scriptures specifically describe.
Furthermore, there is no indication in the scriptures that it had ever rained before (Noah's explicatives may have been censored out)
Or, rather you are making assumptions about the laws of refraction for which you can not support....unless you consider them to be universal, eternal, and infinite....do you?
That being said, aren't you really missing the point of the story?