There is also a zoomable version on the Google Art Project, but it’s quite dark, as is the one on the Dulwich site. Presumably they’re the same.
I’ve never seen the original, so it’s difficult to judge the true nature of the darkness and color of the image. However, my experience has been that museums often display images of the paintings in their collections that are much darker than the original. This may be deliberate, I don’t know.
The Wikimedia image, though credited to the Google Art Project, has apparently been brightened by someone along the way. This usually results in overbright, unrealistic images, but in this case — if someone altered this, they knew wht they were doing.