Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
I don't remember seeing any rainbows in this movie, but there is a serpent. This is a horror movie made by Wes Craven (who made Nightmare on Elm Street, Scream, etc.) It's based on a 1985 book with the same name written by Wade Davis about his experiences in Haiti searching for the secret of zombification. In the movie, Haitians run around smearing chicken blood on each other and blowing powder in people's faces, zombies burst out of coffins, and the chief of the secret police is a voodoo witch who controls zombies with his mind. He also makes life on the island a nighmare for Harvard scientist Dennis Alan who was hired by a pharmaceutical corporation to go to Haiti and find the secret to making zombies, people who are dead according to medical tests but come back to life the next day.
After his previous trip to the Amazon where a shaman drugged him, Dennis has been having frightening hallucinatory dreams. When sleeping at night on a religious pilgrimage with Marielle, Dennis' Haitian doctor girlfriend, he falls asleep and starts hallucinating. We see a large snake crawl over Marielle's body towards him. He wakes up in fear but then a corpse skeleton in a white bridal gown shows up and when he opens her veil, she pulls her mouth open and a giant snake shoots out and bites him on the face. He struggles with it, then pulls it all the way out of her, then wakes up, just in time to have even worse dreams later in the movie.
The snake we see onscreen is a live Gaboon Viper, a venomous snake from Africa. Normally the actors would never be in the same shot with a venomous snake, so I have to assume that when we see the camera pan from right to left across the body of Marielle, that it is a dummy we see and not the actress who plays her, Cathy Tyson. But if it's a dummy, it's a very realistic one. It's possible it really was the actress and that they either sewed the snake's mouth shut or removed its venom glands in order to make sure there was no risk to anyone. My first impression was that the snake was fake, maybe even a computer effect, but after watching it again I think it was real, because you can see the tongue flicker in and out. The end credits list a snake and insect handler which means that there was a live snake used on the set, but it could have been cut out of the movie and the viper could still be fake. There is also a disclaimer stating that all animals were under the supervision of a vet and were not harmed, but maybe the vet doesn't consider surgery on a snake to be harmful. The only thing I do know for certain is that the snake that shot out of the corpse's mouth was definitely a fake snake.