Accessing Resources in Other Assemblies

<SolidColorBrush
x:Key="{ComponentResourceKey TypeInTargetAssembly={x:Type local:MyClass},
ResourceId=MyClassBrush}">Yellow</SolidColorBrush>

<Button Background="{DynamicResource {x:Static otherAssembly:MyClass.MyClassBrushKey}}"/>

WPF unleashed, page 256