First of all, I don't like colored backgrounds. Just a personal bias, but I don't like pics of snakes on blue aquarium gravel. It does show off their colors, I just don't really like it. The problem with most of your choices is not the color, but rather the shinyness (?) of the material. The reflection off this material is taking away from the beauty of the snake.
I have a 3'x3' piece of black "velveteen" that I picked up at walmart for about $1 that I use sometimes. You have to turn it the right direction to avoid the material reflecting, but you can get some nice pics with it. With a little tweaking (darkening) in photoshop or similar program, you can even make the background "dissappear" -


Given a choice, I much prefer a more naturalistic substrate. Here's a young maurus rainbow boa shot on a desktop with a few leaves used as a substrate. I always like that "you could find one like this in the wild" look.

I guess the best suggestion is to look at all the great rainbow pics posted here, look at the substrates, decide which ones you like, and try and emulate that!
-----
Chris Harrison
San Antonio, Texas