Trouble with color exchange

[astronaut]

I am very new to Gimp, and my goal with this image is to change the brick red to fuschia. So far, I have Indexed the Image Mode to 32 colors max, and I am having trouble with using Color Exchange. I chose the "From color" from the image itself, and I want the "To color" to be fuschia. I have tried messing with the red/green/blue thresholds, but I think I am missing something. 

Thanks in advance!

[denzjos]

If you want the whole building give a fuschia color, I suggest to select the surfaces of the building you want to change and use 'Colours / Hue-Saturation'
PS : the green line on the right side of the building is because I've made the selection outside the buiding so you can see what the result is if the selection of the building is not correct. So pay attention with this workflow to make a good selection.

   

[tmanni]

With the Indexed Mode, the image has a limited set of colors ; you can see them with Windows > Dockable Dialogs > Colormap.
If the "fuschia" color is not in the colormap, Color Exchange will use the nearest color available in the colormap.

[rich2005]

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