Color exchange not working

[Blighty]

(11-21-2019, 09:43 AM)She Wrote: I really can't work out why the colour exchange is not working.

You asked about Colors > Map > Color Exchange
but your image is using Colors > Threshold
(Threshold converts the image to pure black and pure white.)

But for that image Colors > Map > Rotate Colors would be better.
(Split view is enabled.)