You can do this with a Worksheet Change event in VBA. I started building a simple example for you, but I realized your customer will probably want to be able to roll back any highlighting after they've checked your changes. So, I decided to make a whole working model that does everything. Here are the steps you'll need to follow:
Press Alt+F11 to open the VBA editor in Excel. In the Object Browser (On the left side of the VBA window), double click a sheet you will be editing. Paste the following code into the text field that opens.
Private Sub Worksheet_Change(ByVal Target As Range)
Dim ws As Worksheet, ws2 As Worksheet
Dim i As Boolean
Application.ScreenUpdating = False
'Create Change Log if one does not exist.
i = False
For Each ws In ThisWorkbook.Worksheets
If ws.Name = "Change Log" Then
i = True
Exit For
End If
Next ws
If Not i Then
Set ws2 = ThisWorkbook.Worksheets.Add
ws2.Visible = xlSheetHidden
ws2.Name = "Change Log"
ws2.Range("A1") = "Sheet"
ws2.Range("B1") = "Range"
ws2.Range("C1") = "Old Text Color"
Else
Set ws2 = Sheets("Change Log")
End If
'Store previous color data in change log for rollback.
ws2.Range("A1").Offset(ws2.UsedRange.Rows.Count, 0) = Target.Worksheet.Name
ws2.Range("B1").Offset(ws2.UsedRange.Rows.Count - 1, 0) = Target.Address
ws2.Range("C1").Offset(ws2.UsedRange.Rows.Count - 1, 0) = Target.Font.Color
'Change font color to red.
Target.Font.Color = 255
Application.ScreenUpdating = True
End Sub
This code will change the font color to red of any cell you edit the value. It will not change the font color if you only change the format of a cell. Also, it will not automatically change the font color of dependent cells. The latter could be done, but I'll leave that to you if you want that behavior.
This code also creates a hidden Change Log sheet and records the address and original font color of the cell that has been changed.
Note that this code only works for changes in that specific sheet. If you want to track changes in the entire workbook, you can paste this same code in the worksheet module for each sheet.
The code to rollback any highlighting must be placed in a separate module. In the VBA editor, go to Insert >> Module. Paste the following code into the new module.
Sub rollbackHILITE()
Dim sht As Worksheet, cl As Worksheet
Dim j As Long, roll() As Variant
Dim del As Integer
Application.ScreenUpdating = False
'Find Change Log. If it doesn't exist, user is prompted and exits sub.
For Each sht In ThisWorkbook.Worksheets
If sht.Name = "Change Log" Then
Set cl = sht
Exit For
End If
Next sht
If cl Is Nothing Then
MsgBox "Change Log not found!"
Exit Sub
End If
'Return font colors to original form by stepping backward through change log.
If cl.UsedRange.Rows.Count > 1 Then
roll = cl.Range("A2:C2").Resize(cl.UsedRange.Rows.Count - 1, 3)
For j = UBound(roll, 1) To 1 Step -1
Set sht = Sheets(roll(j, 1))
sht.Range(roll(j, 2)).Font.Color = roll(j, 3)
Next j
End If
Application.ScreenUpdating = True
'Prompt User to keep or delete change log after rollback.
del = MsgBox("Delete Change Log?", vbOKCancel, "Finish Rollback")
If del = 1 Then
cl.Delete
End If
End Sub
This code can be run from the Macros menu in Excel. It simply reverts the font color of highlighted cells back to their original colors and then deletes the change log if the user OKs it.