Changing one color

[Blighty]

Method will depend on the actual image. Two different types of images are 1) an image created by vector graphics which has pure colours and 2) a jpg which has a large range of colours.

One Method:
Make a selection on the colour to be replaced. Bucket fill with the required colour.

Another Method:
1. Create a transparent layer below the image.
2. On the image layer, make a selection of the colour to be replaced. Delete the selection.
3. Grow the selection by 1 or 2 pixels. Do Colour to Alpha, using the colour that is to replaced.
4.Switch to the lower transparent layer
5. Grow the selection by 1 pixel
6. Bucket fill the selection with the required colour - this still on the lower transparent layer

Yet Another Method
Use Colour > Map > Rotate Colours