In a Silverlight application, Create a resource in App.xaml. Something like this…

    <Application.Resources>
        <RadialGradientBrush x:Key="GreenRadialBrush" >
            <GradientStop Color="DarkGreen" Offset="0" />
            <GradientStop Color="White" Offset="1" />
        </RadialGradientBrush>
        <LinearGradientBrush x:Key="SampleLinearBrush" >
            <GradientStop Color="#443344" Offset="0" />
            <GradientStop Color="#778899" Offset="1" />
        </LinearGradientBrush>
    </Application.Resources>

Go back to mainpage.xaml and draw a rectangle or any control for that matter. Switch to properties for the control and look for Fill (or Brushes -> Fill). Right click on Fill and select Apply Resource

Notice, how it shows all the Resources and the preview as a swatch!

 

Cheers,
Rahul

Quote of the day:
The scientific theory I like best is that the rings of Saturn are composed entirely of lost airline luggage. - Mark Russell