The GanttView control consists of two grids separated by a resize-able splitter.

You can change the color of the splitter by using the SplitterColor property of the C1GanttView class.
Use the below code snippet to modify the color of the splitter in the GanttView.

//スプリッタの色を設定します。
c1GanttView1.SplitterColor = Color.Red;