A rainbow is to be used as a bridge. But one must go under it and not over it. Whoever goes over it will fall and be killed. Only the gods can walk rainbow bridges in safety; mere mortals fall and meet their death
Jung interprets the dream-image of the rainbow as a threshold symbol delineating the boundary between human and divine registers, prohibiting literal ascent while directing psychic movement downward, toward water and unconscious depths.
, Psychology and Alchemy, 1944thesis