The Silverlight System.Windows.Media.Colors class is a trimmer counterpart of the WPF Colors class. E.g. Colors.AliceBlue is available in WPF but not present in Silverlight. However, these standard colors are indeed available in Silverlight XAML.
In effect in Silverlight XAML you can use
<Border Background="AliceBlue" BorderBrush="Coral" BorderThickness="2" CornerRadius="10" >
However, if you try that in code it fails to compile
foo.Background = new SolidColorBrush(Colors.Coral);
This means if you ever need to use standard colors in Silverlight code you have to use RGB values, which for Coral would be something like
foo.Background = new SolidColorBrush(Color.FromArgb(255,255,127,80));
This is kind of painful. Moreover, you need to frequently be able to convert between HTML color, RGB and XAML standard color names when you are developing an Silverlight application. To make the job easier I hacked up a small Silverlight 2 app at http://www.bonggeek.com/Ag/Colors/. You can do the following with this
Enjoy and extend as you please. Sources are available here.