Here ya go:
<Canvas>
<Ellipse Canvas.Left="50" Canvas.Top="50" Width="300" Height="300" Stroke="Black" StrokeThickness="1" Fill="Red" />
<Ellipse Canvas.Left="50" Canvas.Top="50" Width="300" Height="300" Stroke="Black" StrokeThickness="1" Fill="Yellow">
<Ellipse.Clip>
<RectangleGeometry Rect="0 100 300 300" />
</Ellipse.Clip>
</Ellipse>
<Rectangle Canvas.Left="50" Canvas.Top="150" Width="300" Height="200" Stroke="Black" StrokeThickness="1" Fill="Transparent">
<Rectangle.Clip>
<EllipseGeometry Center="150, 50" RadiusX="150" RadiusY="150" />
</Rectangle.Clip>
</Rectangle>
</Canvas>
The first element of the canvas draws a red circle, the second element draws a yellow circle on top and clamps it, the third element draws a black line through the middle. Here is the result:

EDIT: Actually, thinking about this, two more ellipses can be made with a single ellipse and a linear gradient brush:
<Ellipse Canvas.Left="50" Canvas.Top="50" Width="300" Height="300" Stroke="Black" StrokeThickness="1">
<Ellipse.Fill>
<LinearGradientBrush StartPoint="0,0" EndPoint="0,1">
<GradientStop Color="Red" Offset="0.3333" />
<GradientStop Color="Yellow" Offset="0.3333" />
</LinearGradientBrush>
</Ellipse.Fill>
</Ellipse>
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