An alternative approach, which doesn't require you to copy the existing data to a new location, is to add a macro like the following to the worksheet's code module:
Code:
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
With this macro, simply changing any cell's contents will change its background colour to yellow.