If you want to set different colors to different task you may use either the highlighting feature of the GanttView, demonstrated in this example
(it can be customized much more than demonstrated in the example) or to create a custom style for the EventContainer and to bind their background to some property of their GanttTask (which in particular could be calculated from the Parent and common for all siblings). For using different colors for different tasks, please refer to the attached example.
Hope this helps.
All the best,
the Telerik team