The earthquake configuration dialog should receive a groupbox that allows to edit the colors for the different severities.

http://en.wikipedia.org/wiki/Richter_magnitude_scale

Minor, Moderate and Major earthquakes should each receive a different color configuration:

http://en.wikipedia.org/wiki/Richter_magnitude_scale#Richter_magnitudes

See the coordinate grid configuration dialog for examples 

Have a look at

http://techbase.kde.org/Projects/Marble/OnlineServices

and at the existing Online Plugins. The Wikipedia plugin has got a configuration dialog  that you can use as a "template".