A rainbow in a dream typically arrives after some form of hardship, in the dream or in your waking life, and often signals hope and a sense that reconciliation is possible. Seeing one appear right after a storm can point to relief following a difficult period, a reward you feel you've honestly earned.
The Scribe should be asked for manifestation of the hidden, study of mysteries, unfastening of the tongue, circulation of wealth, defamation and transmission of slanders. - Picatrix (Andalusia, ~1000.AD)