You can try using something like:

this.ultraChart1.ChartDrawItem += new Infragistics.UltraChart.Shared.Events.ChartDrawItemEventHandler(ultraChart1_ChartDrawItem);

…

void ultraChart1_ChartDrawItem(object sender, Infragistics.UltraChart.Shared.Events.ChartDrawItemEventArgs e)

        {

            Box box = e.Primitive as Box;

            if (box != null && box.Value != null)

            {

                double value = Convert.ToDouble(box.Value);

                if (value < 40)

                {

                    box.PE = new PaintElement(Color.Red);

                }

            }

        }