As previous, an indexed image assigns the pixels in an image according to a colormap not individually like a RGB image.
It is possible to edit the colormap replacing one colour with another, all pixels assigned will change. It can work but also can be tedious. example: https://i.imgur.com/VRDdMUN.jpg
Unlikely to get any result using color exchange with an indexed image, adjust sliders and see what happens.
Does the image need to be Indexed? Better results with RGB: examples: https://i.imgur.com/8JYybZj.jpg
There are other tools also in RGB mode, Color Rotate, and in Tools -> GEGL Operation ColorWarp (takes a bit of practice initially & not sure if it is there in MacOS) https://i.imgur.com/T01Zeha.jpg
It is possible to edit the colormap replacing one colour with another, all pixels assigned will change. It can work but also can be tedious. example: https://i.imgur.com/VRDdMUN.jpg
Unlikely to get any result using color exchange with an indexed image, adjust sliders and see what happens.
Does the image need to be Indexed? Better results with RGB: examples: https://i.imgur.com/8JYybZj.jpg
There are other tools also in RGB mode, Color Rotate, and in Tools -> GEGL Operation ColorWarp (takes a bit of practice initially & not sure if it is there in MacOS) https://i.imgur.com/T01Zeha.jpg