I have two pictures of the same person. Pictures were taken at same place but slightly different light were used in each one. I would like to adjust the skin color in one picture to match the other so both have the same skin (background is not so important, it can be cropped in the final image containing just the entire head)
*Update: This is what I've tried so far (with poor results)
Image source: https://en.wikipedia.org/wiki/File:Emmy_Rossum_2011.jpg licensed under the Creative Commons Attribution-Share Alike 2.0 Generic license
I did this in photoshop but you can use the same technique in GIMP.
Directions are written for GIMP
Add a curves layer, In the image menu through Tools â†’ Color Tools â†’ Curves or Colors â†’ Curves. You want to brighten up the dark areas without adjusting the shadows too much.
Change the adjustment layer blending mode to Color Mode (Suggestion made by Ryan, luminosity is a blending mode in photoshop).
Note: I used a layer mask to hide the parts I didn't want affected by the adjustment layer.
I added a levels adjustment layer to fine-tune the contrast, In the image menu through Tools â†’ Color Tools â†’ Levels.
Result is a modified version of your top most image.