When I went to school (eons ago) we were taught there were 7 colours in the rainbow.
In church I was taught it is because 7 is a number that often comes up in the Bible and depicts a number of perfection or completion. There are 7 days in the week, for instance, which began all the way at the time of creation...and there are 7 churches mentioned in the book of Revelation, as well as 7 seals, 7 trumpets, and 7 bowl judgments.
Studying numbers mentioned in the Bible and what context they are used in is a fascinating study. Nowadays the popular way to depict a rainbow is with 6 colours, leaving out the colour of indigo. In the Bible the number 6 is the number of man. Humans were created on day 6 of creation. The "man of sin" aka the antichrist is going to be recognized by his number 666 (a trilogy mocking the triune God). But if you click on the image link below the rainbow they give another reason for the 7 colours, and state that it is an arbritrary number that can be divided into millions of hues.
The Bible tells us that the rainbow was given to us by God as a sign of a promise from God, a covenant, that He will never again destroy the earth with a flood of water. (Genesis 9:12-15)
But how do we see that covenant sign displayed today? It is used as a sign of open rebellion, of poking God in the eye saying my sin is my choice and I choose to sin without shame nor fear of judgment.
I saw this meme recently and wholeheartedly agree:
Amen. If the eyes of your understanding are open you can see this is true.
With all of the rain we have been experiencing it is a good reminder of the fact that we worship a loving and caring God who does not enjoy wiping out the earth, and He revealed that fact with the promise of not repeating the total destruction by water again....but the Bible also says there will come a day in the future where the earth will be judged and destroyed but this time it will be by fire. Fire purifies and refines. God will accomplish all that He promises, but through mercy and grace He gives signs and warnings before He sends that cleansing destruction. I pray all who read this will heed the warnings and be "wise virgins" before the coming of the "great and terrible day of the Lord.
For more interesting facts about the rainbow, check out this link: