The Rainbow

The Rainbow is a Biblical Symbol. 

The Rainbow is God's Covenant with Noah and his descendants. 

Genesis 9:8-17

8 Then God said to Noah and to his sons with him, 9 "Behold, I establish my covenant with you and your offspring after you, 10 and with every living creature that is with you, the birds, the livestock, and every beast of the earth with you, as many as came out of the ark; it is for every beast of the earth. 11 I establish my covenant with you, that never again shall all flesh be cut off by the waters of the flood, and never again shall there be a flood to destroy the earth."12 And God said, "This is the sign of the covenant that I make between me and you and every living creature that is with you, for all future generations: 13 I have set my rainbow in the cloud, and it shall be a sign of the covenant between me and the earth. 14 When I bring clouds over the earth and the rainbow is seen in the clouds, 15 I will remember my covenant that is between me and you and every living creature of all flesh. And the waters shall never again become a flood to destroy all flesh. 16 When the rainbow is in the clouds, I will see it and remember the everlasting covenant between God and every living creature of all flesh that is on the earth." 17 God said to Noah, "This is the sign of the covenant that I have established between me and all flesh that is on the earth."