Press Alt + F11 to get to the Visual Basic Editor
On the left hand side, find the file you're working on, and double
click on ThisWorkbook
Its code window will open up.
Insert the following lines of code:

Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As
Range)
Target.Font.ColorIndex = 3
End Sub

Now, whenever you edit a cell, its font color will become red.