Pixiepew, rainbow topaz is a clear topaz that is treated to have that rainbow color. It is a natural stone but the color is not natural. Most of the stones in the market are treated to improve their color, and rainbow topaz is treated to create color.
YW. I'm mentioning it because I too used to think it was a natural color. So when people ask if it's genuine, they might be thinking the color is natural too.
Since the color is created, genuine in this case only applies to whether the treated stone is natural. Although I'm not sure it matters whether it's a natural clear topaz or glass. The effect is the same (in color).