You can try using:

this.UltraChart1.ChartDrawItem += new ChartDrawItemEventHandler(UltraChart1_ChartDrawItem);

…

private void UltraChart1_ChartDrawItem(object sender, ChartDrawItemEventArgs e)

    {

        Box box = e.Primitive as Box;

        if (box == null)

        {

            return;

        }

        if (string.IsNullOrEmpty(box.Path))

        {

            return;

        }

        if (box.Path.EndsWith("Legend") == false)

        {

            return;

        }

        // this is the external legend box

        if (box.Column == -1)

        {

            return;

        }

        // you can use "box.Column" and "box.Row" to determine which series the box belongs to

        Color currentFill  = box.PE.Fill;

        box.PE = new PaintElement(Color.White, currentFill, Infragistics.UltraChart.Shared.Styles.GradientStyle.Vertical);

    }