Brighten up dark photo colors with Gimp

Duplicate the original image layer in the layers dialog. Change the Mode of this duplicated layer to Value -mode to use this layer for a retouching brightening of colors. Further editing of the image, without this masking layer , could lead to a change of original hue and colors within the picture.

Apply the Tools » Color » Levels -dialog to the retouching layer for manual or automatic correction of brightness, color and contrast. Click on the Auto Correction -Button in the levels Dialog to try Gimp's automatic correction of brightness, color and contrast.

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