Red, Orange, Yellow, Green, Blue, Indigo, Violet.
Genesis 9:13-15 I do set my bow in the cloud, and it shall be for a token of a covenant between me and the earth. And it shall come to pass, when I bring a cloud over the earth, that the bow 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; and the waters shall no more become a flood to destroy all flesh.
Green is central because the dove returned bearing a leaf. Not a bluebell, not a sunflower. A leaf. When people reject God they don't just throw out a hue from the rainbow. They throw out everything.
Genesis 8:11 And the dove came in to him in the evening; and, lo, in her mouth was an olive leaf pluckt off: so Noah knew that the waters were abated from off the earth.