Exporting from sketchup, the color dome colors are determined by the background & sky colors of the current style (go to style > background settings), with illumination being handled by a white color dome (otherwise there would be a color cast over the whole scene).
If you look at the exported file in the bella gui, you’ll find this is actually done with an image dome, where all channels are being overridden, with one of the two color domes mentioned above. You can directly set the horizon & zenith colors of a color dome in the gui.
As for making the render darker, if you mean you want less light from the environment, you can dial back the luminance or multiplier for image & color domes. There’s no way to reduce the light from a sky dome besides setting the time to something closer to dawn or dusk.
However you can also change the exposure of the camera; the easiest control for this is camera -> Exposure > EV, where higher EV numbers are suitable for scenes with more light, and lower for scenes with less.