I have a derived DataGridColumnStyle. From within my Paint override, how can I get at other values in the DataGrid

You can get a reference to the DataGrid with code
such as:

            Dim grid As DataGrid = Me.DataGridTableStyle.DataGrid

Once you have the grid, you can use an indexer to get the value of any
particular column on the same row.

            Dim someValue as Object = grid(rowNum, 5)  ’ value in column 5....