sorry about that, please try this link now: new link i’m guessing gimp has similar color adjustment functions. in lightroom for example, you can select a base gray color anywhere in the image (in this case it would be the gray tone in the calibration panel inside the render) and it will auto color adjust the entire image. in photoshop there should be a similar workflor however I haven’t done this in PS since a while. basically, you tell gimp: these are base values for red, green, blue etc, and it figures out the rest of the color values for the entire image.