God keeps His promises.
After the flood, God made a covenant with Noah and every living thing.
Genesis 9:11 (KJV): And I will establish my covenant with you; neither shall all flesh be cut off any more by the waters of a flood; neither shall there any more be a flood to destroy the earth.
The rainbow is God's sign of that covenant. It belongs to Him, not anyone else.
Genesis 9:13 (KJV): I do set my bow in the cloud, and it shall be for a token of a covenant between me and the earth.
Thousands of years later, that promise still stands. God does not lie.
Numbers 23:19 (KJV): God is not a man, that he should lie; neither the son of man, that he should repent: hath he said, and shall he not do it? or hath he spoken, and shall he not make it good?