The nonworking time denotes the time duration during which no work is being carried out, like company holidays, weekends, and so on.

You can change the color of nonworking time line by using the NonworkingTimeColor property of the C1GanttView class.

Use the below code snippet to change the nonworking time color in the GanttView.

//非稼働時間を設定します。
c1GanttView1.NonworkingTimeColor = Color.Blue;