Access Checkboxes() backColor

Hi @ll!
I´ve a form where you can click the checkbox "kostenpflichtig" (means: liable to pay costs). When you mark it, the field (Bezeichnungsfeld148) where the text "kostenpflichtig" appears, should be change from bgcolor "white" into e.g. "red". Alternatively if checkbox is not marked it should be stay white. What I have made wrong?
With tis code nothing happens.. :-(

Private Sub kostenpflichtig_AfterUpdate()
If Me![kostenpflichtig] = True Then
Me![Bezeichnungsfeld148].BackColor = RGB(201, 105, 50)
End If
End Sub

Thx4Uhelp!