Graham has already given you the code for changing the cells as the user makes selection (and exits the cell). It is in the ThisDocument module. I would have written it like this

Sub Document_ContentControlOnExit(ByVal oCC As ContentControl, Cancel As Boolean)
  If oCC.Type = 3 Or oCC.Type = 4 Then
    Select Case oCC.Range.Text
      Case "Y": oCC.Range.Cells(1).Shading.BackgroundPatternColor = wdColorGreen
      Case "N": oCC.Range.Cells(1).Shading.BackgroundPatternColor = wdColorRed
      Case "NA": oCC.Range.Cells(1).Shading.BackgroundPatternColor = wdColorBlue
      Case Else: oCC.Range.Cells(1).Shading.BackgroundPatternColor = wdColorWhite
    End Select
  UpdateTotals
  End If
lbl_Exit:
  Exit Sub
End Sub
When you don't understand the purpose of something. Select it e.g., DoEvents and press F1. That opens any associated help.

lbl_Exit is just a label. I use it and so does Graham. It is a coding style. When actually serving a purpose it may look like this:

Sub Demo()
 Dim oRng As Range
 Set oRng = ActiveDocument.Range
 On Error GoTo lbl_Err:
 'Force and error for demo
 Err.Raise 6
lbl_Exit:
  Set oRng = Nothing
  Exit Sub
lbl_Err:
  MsgBox Err.Number & " " & Err.Description
  Resume lbl_Exit
End Sub