In the poster of 1997 movie Good Will Hunting Robin Williams' character Sean Maguire has a dark beard compared with the movie itself:
Why is there this difference?
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As you can see from the publicity shot below, his beard, at the time the photo was taken, is very much the same colour as you see in the rest of the film.
It's not uncommon for images to be altered for effect for movie posters, in this case presumably to make Sean look slightly younger, and hence more attractive to their target audience.