the Color Exchange tool doesn't work well for me, too.
But there's an easier way:
1. Use the Select by Color tool to select the red areas.
If not everything is selected at the first click, increase
the Threshold value in the dialog.
2. Fill the selected area with plain black either with
the Bucket Fill tool or by dragging it from the
Foreground/Background color swatch in the toolbox.
(If your foreground or background color is not black,
then press 'd' first).
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