Iris (rainbow in Greek) is the rainbow deesse and messager of the gods. She is charged to deliver the divine messages to the mortals. The rainbow is the Iris manifestation ; it is the bridge between the earth and the sky. To the poets, the rainbow is the track of the Iris’s foot when she descends quickly from the Olympus to the earth. They also name the rainbow as the Iris’s scarf.
Sources : http://mythologica.fr/grec/iris.htm and http://moniris.free.fr/deesse.htm