I’m not sure what you’re expecting for results, or what you think the “problem” is.
I’m no where near an expert in radiosity, nor do I have a clue how Carrara’s version is implemented, but I think you need to step back a bit and consider what happens in the real world. As Holly accurately said, it’s not designed to be a physics emulator, and there’s no guarantee the results will accurately reflect the real world, but the results are probably closer than you might think.
First consider how lights and materials operate in the real world. If, for example (and I’m not saying this is the case you’re discussing), you have a completely white light in a completely white room shining on a red floor, the ONLY light bouncing around the room will be red. That’s because the white light hits the the floor, takes on the red color of the floor, and becomes red light, which then colors every white wall it hits with red. Even the spot on the floor hit by white light turns red.
Now, much of this depends on a lot of factors.
The properties of the floor and walls, for example. If they are non-reflective or very bumpy, etc., less light will be bouncing off the floor, and therefore the room will be darker, and, in this case, less red. The properties of the materials involved are hugely important when considering how real light will travel.
Where is the light pointing? If it’s pointing at a white wall, the “color bleed” as some seem to want to call it will be less because only reflected light, not direct light, hits the red floor first.
It’s also a function of color saturation, and relative colors of the floor and walls and light. As well as intensities.
And if the only color present in any lights or materials in your scene are, say, red, then what do you expect? Of course the light in the room will be colored red. Radiosity takes your white light from your light and bounces it a bunch of times off the red floor. Of course you’ll get a lot of “color bleed”.
Now, I’m not saying that the Carrara radiosity is correct or not correct. I’m merely saying it’s really, really complicated, and before you say it doesn’t look right you have to know what “right” is first. And I suspect it’s less a function of which dials you spin and which boxes you check than whether your expectations are in line with the real world.