What type of dropdown are you using? Word doesn't have a built-in "change" event for content control dropdowns, but you could use the OnEntry and OnExit events:
Code:
Dim oCell As Cell
Private Sub Document_ContentControlOnEnter(ByVal ContentControl As ContentControl)
Select Case ContentControl.Title
Case "Demo"
Set oCell = Selection.Cells(1)
End Select
End Sub
Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
Select Case ContentControl.Title
Case "Demo"
Select Case ContentControl.Range.Text
Case "A": oCell.Shading.BackgroundPatternColorIndex = wdBrightGreen
Case "B": oCell.Shading.BackgroundPatternColorIndex = wdDarkRed
End Select
End Select
End Sub