SelectedAppointmentsChangedイベントでTagを設定し、BeforeAppointmentFormatイベントで選択した予定の背景色を変更します。

◆サンプルコード
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Private Sub C1Schedule1_BeforeAppointmentFormat(ByVal sender As Object, ByVal e As C1.Win.C1Schedule.BeforeAppointmentFormatEventArgs) Handles C1Schedule1.BeforeAppointmentFormat
　　If C1Schedule1.SelectedAppointments.Contains(e.Appointment) Then
　　　　e.BackColor = Color.Blue
　　Else
　　　　e.BackColor = e.Appointment.Label.Color
　　End If
End Sub

Private Sub C1Schedule1_SelectedAppointmentsChanged(ByVal sender As Object, ByVal e As C1.Win.C1Schedule.SelectedAppointmentsChangedEventArgs) Handles C1Schedule1.SelectedAppointmentsChanged
　　For Each app As Appointment In C1Schedule1.DataStorage.AppointmentStorage.Appointments
　　　　If C1Schedule1.SelectedAppointments.Contains(app) Then
　　　　　　app.Tag = "change"
　　　　Else
　　　　　　app.Tag = Nothing
　　　　End If
　　Next
End Sub
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