Свойство LabelForeColor объекта Datapoint должно работать
<asp:Series Name="Default" BorderColor="180, 26, 59, 105" ChartType="StackedColumn" >
<Points>
<asp:DataPoint XValue="1" YValues="10" LabelForeColor ="Black" Label="123" />
</Points>
</asp:Series>
<asp:Series Name="Default1" BorderColor="180, 26, 59, 105" ChartType="StackedColumn" >
<Points>
<asp:DataPoint XValue="1" YValues="10" LabelForeColor ="Black" Label="123" />
</Points>
</asp:Series>