You can erase colors by clicking the color button at the top, and select for that color position a color already in use. Colors will be merged.
After doing so, 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.
If the color is used nowhere else in the design, but in the color reduction of that image only, you can also remove it in the color selection of the image editor.
Cliparts may have a different behaviour. There the color positions are not merged. See also FAQ regarding clipart types.