Datagridview _CellValidating question

I have a datagridview and in the _CellValidating event I am checking a cell to make sure it is filled in. If it is not, I want to stop editing, display a msgbox, and highlight the particular cell. Here is my code :

Code:

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If e.ColumnIndex = 2 Then
    If e.FormattedValue = "" Then
        If Me.dgvFish.IsCurrentCellDirty Then
            MsgBox("Sample Number is a required field.")
            Me.dgvFish.EndEdit()
            e.Cancel = True
            Me.dgvFish.Rows(e.RowIndex).Cells(2).Style.BackColor = Color.Aquamarine
        End If
    End If
End If

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For some reason, I can't get the cell's backcolor to be filled in. Any suggestions ? Thanks.

Try replacing 2 with e.ColumnIndex