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.
Sol and Mars are fiery; the heat of Mars is destructive, that of the Sun answers the vivifying heavenly fire. - William Lilly (1602.-1681.)