Genesis 9:13-17
"I set My rainbow in the cloud, and it shall be for the sign of the covenant between Me and the earth. It shall be, when I bring a cloud over the earth, that the rainbow shall be seen in the cloud; and I will remember My covenant which is between Me and you and every living creature of all flesh; the waters shall never again become a flood to destroy all flesh. The rainbow shall be in the cloud, and I will look on it to remember the everlasting covenant between God and every living creature of all flesh that is on the earth.โ And God said to Noah, โThis is the sign of the covenant which I have established between Me and all flesh that is on the earth.โ"
Rainbows weren't visible before Noah's flood to all flesh of the Earth? Because all flesh was colorblind before the flood? Maybe the Nephilim introduced colorblindness to help control the population.