Two people will never see the same rainbow. This is because every human sees colors a different way. The same shade of red you see might be a completely different shade for your brother or sister.
The person who discovered there were seven colors in the rainbow was Sir Isaac Newton. Before Sir Isaac Newton, there were many debates on how many colors there were. Homer said it only had one: purple. Aristotle said three: purple, yellow and green.
Of course, there really aren't seven colors. Each color fades into the next, creating an impressive array.
It was Newton's rival, René Descartes, who, in 1637, discovered rainbows are created from light shining through water droplets.
The word "rainbow" in Latin is arcus iris or arcus pluvius, which pretty much means a “rainy arch”.