Hi Lon Hofman,
Thanks for using Syncfusion product.
In SfSchedule, we have maintained separate opacity value for ReadOnly appointments to show difference from normal appointments. We cannot set solid color for ReadOnly appointments. This is not original behavior of SfSchedule control. We have updating the color for ReadOnly appointments based on ReadOnlyVisibility property but we have maintaining internal setter for this property. But it is possible to set solid color for ReadOnly appointments based on ReadOnlyVisibility property. We have also prepared a sample for the same in which we have set the solid color for ReadOnly appointments using ReadOnlyVisibility property by reflection. Please find the code snippet and sample from the below location.
PropertyInfo propinfo = readOnlyAppointment.GetType().GetProperty("ReadOnlyVisibility", BindingFlags.Instance |BindingFlags.Public);
readOnlyAppointment.AppointmentBackground = new SolidColorBrush(Colors.Red);
Let us know whether this helps also if you need any further assistance on this.