Driving back from dinner [1] Rob [2] and I saw a perfectly formed rainbow across the sky, and just above it, so faint that it could be barly made out, a second one.
12. And God said, This is the token of the covenant which I make between me and you and every living creature that is with you, for perpetual generations:
13. I do set my bow in the cloud, and it shall be for a token of a covenant between me and the earth.
14. And it shall come to pass, when I bring a cloud over the earth, that the bow shall be seen in the cloud:
Genesis 9:12-14 [3]
While in the Western world a rainbow is seen as something good, in certain tribes in the Amazonian basin, rainbows are considered to be an ill omen or of evil. I don't know for sure, but my guess would be that in the jungles, animals that are poisonous to eat show this through bright colors in their skin or plumage, while non-poisonous animals are more earth colors—greens or browns. So things that are brightly colored should be avoided, and because rainbows are brightly colored, they too, bode ill.
[2] http://www.tragic-smurfs.com/