It really depends on the size of the window or door its going up against, the smaller the louvre the less light comes into the room, on most windows people choose either the 63mm or 76mm.
There is a chink of light around the louvres and also the panel, generally I fit a four sided frame and if the cill is out of level I caulk the joints so the only light that comes in is between the shutters and the frame, if by chance you need black-out I can fit a concertina black-out blind behind the shutters.
White will bring the light into the room, colours absorb the light and therefore the room is not so bright, I say don’t try and match your walls or carpet both of which you can change, 99% of houses I fit in have white windows and white window cills, so try and match them.
