[timonoko]

Normalize each color channel individually. You will be astonished how well this works.

  gm convert $1 -channel red -normalize -channel green -normalize -channel blue -normalize normalized/$1

[timonoko]

It is called normalize-like-hunt. Famous Kite Aerial Photographer Alan Hunt suggested doing it manually in Paint Shop Pro. Winter scenes were particularly problematic, as early digital cameras invented crazy colors for snow.

[jcupitt]

Although I think this technique was well known before then -- Crosfield were using channel-independent histogram normalization to restore faded transparencies back in the 80s.