How to change a cell fill color when number value is changed in Excel?

[Rogerswrightonerogers]

Price list is pre-filled with product and cost. However, client may change the cost if needed. 
Looking for formula/formatting so that when cost is changed, the cell fill color changes to alert of change.  CONDITIONAL FORMATTING did not work.
Sample list 

[#2]

@Rogerswrightonerogers 

Maybe with the Worksheet_Change event. In the attached file you can change the product cost in any of the cells in range D2:D20 and the cell is highlighted.

Private Sub Worksheet_Change(ByVal Target As Range)

If Target.Cells.Count > 1 Then GoTo done

If Application.Intersect(Target, ActiveSheet.Range("D2:D20")) Is Nothing Then GoTo done

Application.EnableEvents = False

Target.Interior.ColorIndex = 6

done:
Application.EnableEvents = True
Exit Sub

End Sub