You should set its Stroke, StrokeThickness, Fill, et al. as you would for any Shape, as well as its Width and Height.

A ToolHandle is rectangular by default, but you can set or bind its NodeFigure attached property to get different figures. You can specify a limited number of figure shapes for these handles: NodeFigure.Rectangle (the default figure), NodeFigure.Ellipse, NodeFigure.Diamond, NodeFigure.TriangleRight, NodeFigure.TriangleDown, NodeFigure.TriangleLeft, NodeFigure.TriangleUp, NodeFigure.PlusLine, NodeFigure.XLine, NodeFigure.AsteriskLine,

A simple example definition:

              <DataTemplate>
                <go:SpotPanel>
                  <go:ToolHandle go:SpotPanel.Spot="Center" go:NodePanel.Figure="Ellipse"
                          Width="6" Height="6" Fill="Yellow" Stroke="Black" StrokeThickness="1" />
                </go:SpotPanel>
              </DataTemplate>
            

You can have many tool handles in a single Adornment.

System.Object
   System.Windows.DependencyObject
      System.Windows.UIElement
         System.Windows.FrameworkElement
            System.Windows.Shapes.Shape
               System.Windows.Shapes.Path
                  Northwoods.GoXam.ToolHandle

Reference