And here comes another great tool made by Jed Smith. This one is similar to CurveTool and allows you to track the color of an area of the image, then applies all that information to a Grade node. It is a lot more efficient and intuitive than the native Nuke tool, and it also comes with instructions so it is really easy to follow.
If you want you can see the code (and many other amazing ones from Jed) on his GitHub.
Expand ColorTracker