Option Explicit
Dim NewVal As String, OldVal As String, NewAddr As String, OldAddr As String
 
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
NewVal = ActiveCell.Formula
NewAddr = ActiveCell.Address
If OldAddr <> "" Then
  With ActiveSheet.Range(OldAddr)
    If .Formula <> OldVal Then
      .Interior.ColorIndex = 6
    End If
  End With
End If
OldVal = NewVal
OldAddr = NewAddr
End Sub