Sorry, I did not realise you actuall had a box, I thought that was just your techincal name for the control.
As you mention, they do not have a CF option.
So code as you requested.
Code:
Private Sub Naam_AfterUpdate()
If Not Me.Naam & "" = "" Then
Me.Box1.BackColor = vbRed
End If
End Sub
However you have the backstyle as Transparent, so will never see the colour until you change that.