When I was playing with direct serial access, I sent values between 0-255 for r,g,b. All worked fine. They Python library for rainbow has a brightness value but it does nothing with it.
You could modify the library to scale all the values. But its simpler to just ignore the idea of brightness and just set your RGB values appropriately in your own software.
And yes they're crazy bright. Values up to about 50 are more eye friendly - especially for development where you tend to stare at the things.