<Style TargetType="telerik:SimpleTreeCellContainer">
	<Setter Property="Background" Value="{Binding DataItem.DataItem.Background}" /> <!-- where Background is a custom property a custom GanttTask implementation -->
</Style>

<telerik:RadGanttView.Resources>	
	<Style TargetType="telerik:SimpleTreeCellContainer">
		<EventSetter Event="Loaded" Handler="SimpleTreeCellContainer_Loaded" />
	</Style>
</telerik:RadGanttView.Resources>

private void SimpleTreeCellContainer_Loaded(object sender, RoutedEventArgs e)
{
	var container = (SimpleTreeCellContainer)sender;
	var cellInfo = (CellInfo)container.DataItem;
	var task = (CustomTask)cellInfo.DataItem;
	container.Background = task.Background;
}