DataSeries ds = new DataSeries()
 {
   ValuesSource = new double[] {-2, -1, 1, 2, 2, 1, -1, -3 }
 };

SolidColorBrush red = new SolidColorBrush(Colors.Red);
SolidColorBrush blue = new SolidColorBrush(Colors.Blue);
ds.PlotElementLoaded += (s, e) =>
   {
     PlotElement pe = (PlotElement)s;
     if (pe != null &&
       pe.DataPoint.PointIndex >= 0) // only points
     {
       // set color depending on point value
       if (pe.DataPoint.Value >= 0)
         pe.Fill = red;
       else
         pe.Fill = blue;
     }
   };

c1Chart1.Data.Children.Add(ds);
c1Chart1.ChartType = ChartType.LineSymbols;