When I click the "Channel" dropdown in the Curves tool in GIMP, all of the channels are grayed out and I can only select "Value". What would cause them to be grayed out? What do I need to do to change the color curves of particular color channels?



The channels become grayed out whenever the image is not using the usual RGB mode (in other the words image is grayscale or color-indexed). To change the color mode, click "Image" -> "Mode" -> "RGB". Then you will be able to use the Curves tool to its full extent.

Image Mode RGB

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