Not sure why you are seeing grey bars, particularly on a plasma set that usually produces perfect blacks. Could this be more of an issue withe format recorded on the DVD you are trying to watch? Have you tried a more recent 4:3 film to see if you get the same problem? I don't think you can change the colours of the bars other than play around with the sharpness, brightness and colour settings from the menu but this is not usually advised as the B&O sets are usually set to optimum performance. Perhaps there is a recent software update for the TV that may address the problem? Whatever the case I would suggest going back to the dealer for this query.
Simon.