Genesis presents a rainbow as the sign of God's covenant after the flood and Noah's Ark.
In Genesis 9, God establishes a covenant with Noah, his descendants, and every living creature that the earth will not again be destroyed by a flood of that kind. The rainbow is placed in the clouds as the sign associated with that promise.
The passage describes the rainbow as a reminder of the covenant between God and the earth. It is therefore more than a weather detail in the narrative: it marks a new relationship and a promise concerning the future of living things.
The rainbow also appears in other biblical passages with symbolic significance, but Genesis 9 is the passage most directly connected with Noah's Ark. The text does not call the sign a seven-color system; that familiar description comes from later cultural and scientific traditions.