make the SolidColorBrush appear semitransparent by setting the Opacity property
<UserControl x:Class='SilverlightApplication3.MainPage'
xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'
xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
xmlns:d='http://schemas.microsoft.com/expression/blend/2008'
xmlns:mc='http://schemas.openxmlformats.org/markup-compatibility/2006'
mc:Ignorable='d'
d:DesignWidth='640'
d:DesignHeight='480'>
<Canvas Height="100" Width="200">
<Canvas.Background>
<SolidColorBrush Color="Blue" Opacity=".50" />
</Canvas.Background>
</Canvas>
</UserControl>
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