A rainbow is a curved spectrum of light that appears when sunlight hits moisture in the air. For years the order of visible colors was Red, Orange, Yellow, Green, Blue, Indigo, and Violet (ROY G. BIV — Richard Of York Gave Battle In Vain (at Bosworth Field in 1485, if you can be arsed to look it up)), but in a scientific brainfart the color Indigo was removed from the lis; much like Pluto had its planetary status revoked. Though it should be mentioned that the only reason indigo was there in the first place was that Newton thought that there should be seven colours in the rainbow as the number "7" was considered to be a more perfect number than its mates.
Rainbows are featured in many mythologies. In the Bible, rainbows are a promise from God that He will never flood the Earth again. In Norse mythology, the bridge from Earth (Midgard) to the realm of the gods (Asgard) is a rainbow called Bifröst.
Ronnie James Dio and Ritchie Blackmore formed a heavy metal band named Rainbow in the mid 70s after Blackmore left Deep Purple. Dio would leave Rainbow and join Black Sabbath in the late 70s.