Further update, I created a property on my business object that does the column comparison and then use a bool to brush converter to set the CellValuePresenter background.

Ok so once I change the target type of the style this worked:

      RedStyle.TargetType = typeof(ValueEditor);
      RedStyle.Setters.Add(new Setter(ValueEditor.BackgroundProperty, System.Windows.Media.Brushes.Firebrick));
      RedStyle.Setters.Add(new Setter(ValueEditor.ForegroundProperty, System.Windows.Media.Brushes.Ivory));
      RedStyle.Setters.Add(new Setter(ValueEditor.FontWeightProperty, FontWeights.Bold));

BUT it still doesn't call each time the data changes.

Also, this grid is readonly so it's no that logical that I have to set the EditorStyle - which is why I was playing with CellValuePresenter as it says in the doc.

Actually that is the kind of thing I want to do but it doesn't work:

      _grid.InitializeRecord += new EventHandler<Infragistics.Windows.DataPresenter.Events.InitializeRecordEventArgs>(_grid_InitializeRecord);

      RedStyle.TargetType = typeof(CellValuePresenter);
      RedStyle.Setters.Add(new Setter(CellValuePresenter.BackgroundProperty, System.Windows.Media.Brushes.Firebrick));
      RedStyle.Setters.Add(new Setter(CellValuePresenter.ForegroundProperty, System.Windows.Media.Brushes.Ivory));
      RedStyle.Setters.Add(new Setter(CellValuePresenter.FontWeightProperty, FontWeights.Bold));

    private void _grid_InitializeRecord(object sender, Infragistics.Windows.DataPresenter.Events.InitializeRecordEventArgs e)
    {
      CellFormatters.FormatValue1Cell(e.Record as DataRecord);
    }

    public static void FormatValue1Cell(DataRecord record_)
    {
      if (record_ != null)
      {
        IData item = record_.DataItem as IData;
        if (item.Value1.HasValue && item.Value1 < item.Value2)
        {
          record_.Cells["Value1"].EditorStyle = RedStyle;
        }
      }
    }

This never sets the value to the specified style.

I also tried this:

          CellValuePresenter valuePresenter = CellValuePresenter.FromRecordAndField(record_, record_.Cells["Value1"].Field);
          valuePresenter.Style = RedStyle;

But that doesn't work either because CellValuePresenter.FromRecordAndField always returns null.

Finally, InitializeRecord only gets called once....I need it to evaluate the style every time the data changes.  I can't use a Converter as I need to compare two values and inside the converter you don't have access to the grid/row/cell being updated.