[gazzae]

Stick this into the worksheet change function...


ActiveCell.Offset(-1, 0).Interior.ColorIndex = 3

Whenever a cell is changed that will make the cell red.

You may need to play about with it as if they click on a cell to change it and instead of hitting enter they click on another cell it will change the wrong cell red. But at least it gives you a starting point.
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actually, ignore that, here is how to do it properly.

Put this under worksheet change, it will chage the correct cell.

strrow = Target.Row
strcolumn = Target.Column
Cells(strrow, strcolumn).Interior.ColorIndex = 3