Like the cross, the rainbow has always been a symbol and sign about God's justice, forgiveness, and restoration of sinful people just like me.
And God said, “This is the sign of the covenant I am making between me and you and every living creature with you, a covenant for all generations to come: I have set my rainbow in the clouds, and it will be the sign of the covenant between me and the earth. Whenever I bring clouds over the earth and the rainbow appears in the clouds, I will remember my covenant between me and you and all living creatures of every kind. Never again will the waters become a flood to destroy all life. Whenever the rainbow appears in the clouds, I will see it and remember the everlasting covenant between God and all living creatures of every kind on the earth.”
So God said to Noah, “This is the sign of the covenant I have established between me and all life on the earth.”
Genesis 9:12-17 NIV