Worksheet_change Event Disables Undo

I have a user who wanted a spreadsheet to highlight cells containing formulas whenever the formula was changed. (They want the ability to make changes, but want those changes highlighted. They do not want to protect cells with formulas.) I tried to accomplish this with Conditional Formatting but could not figure out a way to do it, so I turned to VBA. I wrote the following routine that is called on the Worksheet_change event:

Sub CheckFormulas(Target As Range, FormulaRange As String)
Dim CurCell As Range
For Each CurCell In Range(FormulaRange)
If Target.Address = CurCell.Address Then
Target.Interior.ColorIndex = 3
End If
Next CurCell
End Sub...................