One thing you may want to be using is global illumination. When GI is on, the glow channel on an object contributes to the scene as lighting. I don't have Carrara in front of me, but I'm pretty sure you can put in a texture map for the glow channel, to adjut the color. I know you can set the glow to be, say, a multiplier, and put your color in one side and a value in the other, to adjust it.