Try entering the following into your worksheets code module:

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$K$33" Then
If Target.HasFormula = True Then
Target.Interior.ColorIndex = -4142 'automatic
Else
Target.Interior.ColorIndex = 50 'green
End If
End If
End Sub

This will change the interior color of the cell to green whenever the
cell DOES NOT
contain a formula. You can substitute in any kind of formatting you
choose. If you need further help, gimme a holler.....