Colors can be customized inside your sequence diagram.
From the menu bar select Window > Preferences
>Soyatec > eUML2 Modeler > Sequence Diagram > Colors.
Three options are available:
1.Color options
Use fill gradient color
This preference allows you to fill your diagram with gradient color.
Eclipse presentation.
Eclipse presentation if Use of colors for borders is selected in the menu.
This option is available only with "Eclipse presentation style"
2. Color Set
This preference allows you to set color of the different elements which appear in your class diagram
3. Diagram Background Color
This preference allows you to set the bakckground color in your sequence diagram