Before-After
Here is a classic example of clients who tried to coordinate but did not realize that while white and grey "go with" black, the contrast is distracting in a portrait! Here you can see the difference where everyone is dressed in dark grey or black. The eyes focus on the subjects instead of zero-ing in on the white shirt! Changing white or light grey to black is definitely not an easy task, and although I did it to demonstrate this point for my gallery, I would not volunteer to do it for a client. It is extremely time consuming and difficult to make believable, therefore would require an additional art fee. The best answer is to have the client wear anything dark--even if it is purple or navy or green! It is easy to take an already dark color and make it black by desaturating. Making white into black is agony! :)