Rainbow BTA is a name for any generic BTA with more than one color... and usually three. Some vendors have further named them, but most of them are the same and the ones that are not need the lineage back to that vendor to really have the name.
There are some BTAs that command high dollar and the name matters, but rainbows are not one of them. Just buy what you like and enjoy it.
I will caution that some BTAs don't do well with others, of if you catch the bug, then do your homework before you spend a lot on one of them. One of the reasons why some types are so expensive is that they die for a lot of people often in the presense of other BTAs. The more reasonably priced ones don't usually have this issue.