You can accomplish this by using a LinearGradientBrush.
<Rectangle Width="100">
<Rectangle.Fill>
<LinearGradientBrush EndPoint="1,0.5" StartPoint="0,0.5">
<GradientStop Color="#FF2F3243" Offset="1"/>
<GradientStop Color="#FF5B5E6D" Offset="0.5"/>
<GradientStop Color="#FF2F3243"/>
</LinearGradientBrush>
</Rectangle.Fill>
</Rectangle>