I've been looking at passages involving Renly's rainbow guard, the individual members of that group, sigils and seats, people with similar or related names, people with strong opinions about the rainbow guard, green armor and other passages relating to rainbows. There is still a lot to analyze and some of the conclusions I've drawn are more tentative than others, but here are a few things that seem to be true about rainbows in ASOIAF:
Rainbows might be the opposite of shadows. (Hmm. Denys