Alex,

This can be automated unless you are using a proteccted form.  If it a
simple table and you can wait until your data is entered then process
the entire table or column of cells, you can do it with a macro.  Here
is and example that will process the cells in column three of the first
table in a document:
Sub Test()
Dim oCol As Column
Dim oCell As Cell
Dim myRange As Range

Set oCol = ActiveDocument.Tables(1).Columns(3)
For Each oCell In oCol.Cells
 Set myRange = oCell.Range
 myRange.End = myRange.End - 1
 If myRange.Text = "y" Then
   oCell.Shading.BackgroundPatternColor = wdColorGreen
 ElseIf myRange.Text = "n" Then
   oCell.Shading.BackgroundPatternColor = wdColorRed
 Else
   'Do nothing
 End If
Next
End Sub