"Do you know we do not all see rainbows the same way?" asked Celebrimbor. "We elves see seven colours in the rainbow, but many Men see only five. And I have been told that Dwarves see six, but one of them is beyond red and not visible to other races. So Telchar said, in days of old."
"Ha," said Narvi, determined not to be impressed by any talk of dwarven heroes of the First Age today. "Aren't you rather missing the point?"
"It's there, and it's quite something", said Narvi, pointing skywards above the Sirannon.
"Magnificent!" Celebrimbor agreed.
"The Heavenly Arch" is the name of a House of Gondolin. It may be intended to be ultimately the same word as "Ilweran", said to be the Quenya name of the rainbow, although this apparently means literally "bridge of heaven". The name of the house is said to refer to the rainbow, and the members of the house bore a boss on their shields made up of seven gems, probably in allusion to the seven colours of the rainbow, suggesting that the elves distinguish seven colours in it.
Wikipedia says that there is some indication that people tend to see as many colours in the rainbow as they distinguish basic colour terms in their language and that Newton saw five, then revised his account to seven.
(And it might be useful to dwarves to be able to see infrared. I think someone has suggested this in fan fiction before, but I don't remember who.)
Perhaps unsurprisingly, the Tolkien Weekly prompt for this was "rainbow".
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