I personally would prefer to see two colours.
I would make the sign out of fluted PVC the kind of board that I use for estate agents and at say 2’ x 1’6” I would charge £25.00 and for the extra cost in materials to me I would throw the extra colour in free of charge (it adds to your reputation). A job like that would take me about half hour from start to finish, so making £50.00 an hour is OK by me. If the client wanted to see a proof first the price would go up considerably because then you start to get into the grounds of consultation (not good for small jobs)