Blue corns are a new morph, which is actually a grey color, similar to the lavenders only blue hues. They aren't really blue in color. Some breeders are experimenting with this new morph (I'm not sure if it's its own morph yet). I've never heard of rainbows though. One breeder I spoke with told me that they are typically produced from a dilute anerythristic which is selectively bred. You can visit
http://iansvivarium.com/morphguide/diluteanery.html to see a picture of one. However, like I said, they aren't really blue in color, as least as of yet. I am sure with more selective breeding, maybe these guys will develop more of a blue hue to them, like the lavenders are beginning to do.