It is not a formula it is Code [VBA code]!
Open the workbook that you want it to run in.
Hit:
Alt+F11
Hit:
Ctrl+R
Select by double clicking "ThisWorkbook" in the list of Modules in the Project list. Paste a copy of this code, into the Code page that opens.
Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)
'ThisWorkbook module code!
If Target.Value > xlNull Then
Target.Interior.ColorIndex = 36
Else
Exit Sub
End If
End Sub
Then from the toolbar select: Insert - Module and Paste a copy of this code there:
Sub reSetCellColor()
'Standard Module code, like: Module1.
Dim ws As Worksheet
For Each ws In Worksheets
ws.Select
Cells.Select
Selection.Interior.ColorIndex = xlNone
Range("A1").Select
Next ws
End Sub
Click the Upper most upper-right Close "X" to return to your Sheet.
Any changes will now be colored light yellow.
To reset all cell colors back to none:
From the Excel toolbar: Tools - Macro - Macros...
Select the macro and "Run"
Note if wile in the Macro selection Utility you select Options, you can assign a Hot-Key to the Macro, like Ctrl+z
Then to re-set all the cell colors you can just hit: Ctrl+z without needing to go through all the toolbar menus!