- Open the image file
- menu Colors/Map/Color Exchange
- Press the colour selection for "From Color". Set R, G and B to all 255. Press OK button.
- Press the colour selection for "To Color". Set R and G to all 0. Set B to 255. Press OK button.
- Press OK button.
- Open the image file
- menu Colors/Hue-Saturation
- Press "Master" button
- Drag the Hue slider
Source: Portable GIMP 2.8.8