WPF toolkit charting set color of Bar or Pie

Experts,

i m new to charting.i am using the WPF charting toolkit and took help form this blog:

http://www.codeproject.com/KB/WPF/WPFChartingMVVM.aspx?fid=1591697&df=90&mpp=25&noise=3&prof=False&sort=Position&view=Quick&fr=26#xx0xx

in XAML i m using 

 <chartingToolkit:Chart x:Name="ChartPie" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" Margin="5,0,0,0"            

                        Title="{Binding   Path=Title}" LegendTitle="{Binding   Path=LegendTitle}" Visibility="{Binding IsPiechart}" > 

                <chartingToolkit:PieSeries x:Name="XPieSeries" 

                DataContext="{Binding}" DependentValueBinding="{Binding Path=Value}" IndependentValueBinding="{Binding Path=Key}" ItemsSource="{Binding Path=dataPie}" Visibility="{Binding IsPiechart}" />

            </chartingToolkit:Chart>

and in the View model i am  just updating the observalbe collcection "dataPie".

        private ObservableCollection<KeyValuePair<string, double>> _dataPie = new ObservableCollection<KeyValuePair<string, double>>();

        public ObservableCollection<KeyValuePair<string, double>> dataPie

        {

            get { return _dataPie; }

            set { _dataPie = value; }

        }

  dataPie.Add(new KeyValuePair<string, double>("Yes", resultdiv));

  dataPie.Add(new KeyValuePair<string, double>("No", (100 - resultdiv)));

but how can i control the color in the pie chart. every time the chart refreshes the color is changing. please let me know how to control the color.

thanks in advance.