To erase / replace colors
- click the color that should be removed at the top (dark red)
- select a color already in use (green).
- colors will be merged, you have a free color position
After this, it is usually a good idea to open uploaded images in the image editor again, just to make sure this has no unexpected effects on the color reduction of the image.
Cliparts may have a different behaviour. There the color positions are not merged. See also FAQ regarding clipart types.