If you move series up or down in relative order, the colors of the series generally change, because colors are assigned in the order of the series.  However the preferences UI is not updated to reflect this change, specifically Customize Chart -> Style -> Line -> Color needs to be updated every time series order is changed.  Otherwise the chart preview displays a different color than this widget, and to get the correct color in the widget you have to click on another series and then come back to the one that you just moved.

If there are any other automatically determined series properties (e.g. dotted lines when running out of colors if there are high numbers of series maybe?) then these will need to be refreshed too -- not sure how Gnumeric automatically picks series colors.

gnumeric-1.9.16-1.fc13.x86_64
goffice-0.7.16-2.fc13.x86_64