Iris was the fleet footed message of gods, and the personification of the rainbow.

Iris, the daughter of Thaumas and Electra, personified the rainbow, and was the special attendant and messenger of the queen of heaven, whose commands she executed with singular tact, intelligence, and swiftness.

The Rainbow

Rainbows have been long construed as a sign from the heavens, and this is doubtless the reason why Iris, should have been enlisted by the Greeks to be the conduit between gods and men.

Iris Representations

Iris is usually represented seated behind the chariot of Hera, ready to do the bidding of her royal mistress. She appears under the form of a slender maiden of great beauty, robed in an airy fabric of variegated hues, resembling mother-of-pearl; her sandals are bright as burnished silver, she has golden wings, and wherever she appears, a radiance of light, and a sweet odour, as of delicate spring flowers, pervades the air.

…

The first day of every month a ewe-lamb and sow were sacrificed to Hera. The hawk, goose, and more particularly the peacock were sacred to her. Flocks of these beautiful birds generally surround her throne and draw her chariot, Iris, the Rainbow, being seated behind her. {19}

…

The heroes prepared a banquet on the sea-shore, to which they invited Phineus; but no sooner had he taken his place, than the Harpies appeared and devoured all the viands. Zetes and Calais now rose up into the air, drove the Harpies away, and were pursuing them with drawn swords, when Iris, the swift-footed messenger of the gods, appeared, and desired them to desist from their work of vengeance, promising that Phineus should be no longer molested. {221}

From: Berens, E.M. The Myths and Legends of Ancient Greece and Rome. New York: Maynard, Merril, & Co., 1880. Text in the public domain.