This rectangle is painted with a horizontal linear gradient


   
  
<Window x:Class="Workspace.DockExample"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Workspace" Width="640" Height="480">
    <Canvas Width="200" Height="100">
      <Rectangle Width="200" Height="100">
        <Rectangle.Fill>
          <LinearGradientBrush StartPoint="0,0.5" EndPoint="1,0.5">
            <GradientStop Color="Yellow" Offset="0" />
            <GradientStop Color="Red" Offset="0.25" />
            <GradientStop Color="Blue" Offset="0.75" />
            <GradientStop Color="LimeGreen" Offset="1" />
          </LinearGradientBrush>
        </Rectangle.Fill>
      </Rectangle>
      <Line X1="1" Y1="50" X2="199" Y2="50" />
      <Path>
        <Path.Data>
          <GeometryGroup>
            <EllipseGeometry Center="0,50" RadiusX="4" RadiusY="4" />
            <EllipseGeometry Center="50,50" RadiusX="4" RadiusY="4" />
            <EllipseGeometry Center="150,50" RadiusX="4" RadiusY="4" />
            <EllipseGeometry Center="200,50" RadiusX="4" RadiusY="4" />
          </GeometryGroup>
        </Path.Data>
      </Path>
    </Canvas>

</Window>