colors in series

How can I change a column color within a series (one barseries)? Thanks!

The Sample Explorer has an example of one way to do this; by providing a converter that can set the Background of a bar based upon its point's value - look for the Data Dependent Chart Style Demo (SampleExplorer/Demos/Charts/Features/DataDependentChartStyleDemo.xaml).

Essentially what you need to do is create a converter that in its method Convert will return a SolidColorBrush based upon whatever logic you specify. In the above demo we use the ValueToBrushConverter to do this (it's in C# but the concept is the same for VB). In your page's ResourceDictionary create a instance of your converter, then create a Style targeting the Bar type. Inside its ControlTemplate set the Background property of a Border instance to bind to whatever value you want to pass to your converter, then specify that converter. The markup could look something like this:

<Style x:Key="DataDependentBarStyle" TargetType="ch:Bar">
        <Setter Property="Template">
          <Setter.Value>
            <ControlTemplate TargetType="ch:Bar">
              <Border Background="{Binding YObject, Converter={StaticResource ValueToBrush}, RelativeSource={RelativeSource TemplatedParent}}" />
            </ControlTemplate>
          </Setter.Value>
        </Setter>
      </Style>

Then on your BarSeries set its BarStyle to use that StaticResource. I hope I've understood your request, if you want any further clarification feel free to ask.