Thank you for your reply. I tried to find a solution but It seems like sashColor is not supported in 9.0 + OS. In case of Notification View's background color, I reccomend to use custom SwiftUI view for notificationView and WKUserNotificationHostingController instead of WKUserNotificationInterfaceController.

Hello Thanks for your reply. In case of background color of long-look notification View, I recommend to use Custom SwiftUI VIew with WKUserNotificationHostingController than WKUserNotificationInterfaceController. I hope this could solve your problem!