I have, for the past 6 years, been working on techniques of processing photos and other digital artwork using GIMP. As a result of that study I have developed a number of advanced techniques, one of which is using individual layers tuned for specific areas of the image to correct colors.
The first step in the process is to decide what needs to be changed about the image. For this generally I try for color correction on each color or texture of the image, along with any specific areas that need work. Figure 1 shows an example image with different generalized areas highlighted for correction. This step is essential to the success of the rest of the processing, because it allows you to decide what each new layer you work on should do for the image.
After you decide on the sections you then need to make a general outline of what you are going to be doing to each section. Starting with the Red outlined region, I decided to make significant adjustments to the saturation and lightness of this section, primarily to make it look the way it did to me at the time, but also because it looked rather dull as it was. The Yellow region I decided needed a careful attention to make sure that it kept all of the detail, while meshing with the neighboring regions. The Green section needed the tree removed, because of the poor stitching job on it, and other than that I wanted to process it the same as the Magenta sectors. The Cyan section needed contrast and saturation enhancements. Blue needed similar saturation, but no contrast adjustments. Finally the Magenta sections needed to be processed for optimal white balance and contrast.
You can start hard or easy while processing in this fashion, and I tend to start hard, but in this case I started with the easy part. The cloning for the Green section was what I decided needed taking care of right away, because it would make all of the processing layer based, instead of having to finalize a version of it and then do the cloning. I won't go into detail about cloning in this article, but the clone was particularly easy, despite the large area of image because of the almost solid background color. From there I decided to take care of regions Red, Cyan, Blue, Magenta, further adjustments to Red, and some overall tweaks, the whole time balancing Yellow.
In the Part 2 article I will discuss the particular layer modes I used to do the adjustments. Another important skill that I will be covering soon that I used extensively is color correction via the levels and curves tools. For now I will leave you with a screen-shot of the open GIMP file used for this image.