SolidColorBrush's color is specified using 8-digit hexadecimal notation.


   
 

<Window x:Class="WpfApplication1.ShapesWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="ShapesWindow" Height="160" Width="400">
      <Window.Resources>
        <Style TargetType="{x:Type Rectangle}">
          
          <!-- Gives all the rectangles in this panel a white stroke. -->
          <Setter Property="Stroke" Value="White"/>
          <Setter Property="StrokeThickness" Value="1"/>
        </Style>
      </Window.Resources>
    <Canvas>


      <Rectangle Width="50" Height="50">
        <Rectangle.Fill>
          <SolidColorBrush Color="#FF0000FF" />
        </Rectangle.Fill>
      </Rectangle>

    </Canvas>
</Window>