I had a need for this last week, and I thought it may be useful for others. This script shows how to use the curve tool node to sample color data and apply it to something else. This can be super useful for color matching a clip that has wild variations (like camera flashes). It has a color bar node fed into the curve tool and then the color sampled is fed out to a constant.
The one trick with this whole idea is that the crop feature that is built-into the curve tool breaks more frequently than it works. I have fed the curve tool node a cropped image as a way of faking the sampler area.
Click here to download the script.