Nonexistent code, referencing a color, is breaking my app

In my Application resources I have

<Application.Resources>
    <Color x:Key="SMSBlue">#008CFF</Color>
</Application.Resources>

And somewhere in a control template in a Window I have

<ControlTemplate.Triggers>
    <Trigger Property="IsSelected" Value="True">
        <Setter TargetName="Panel" Property="Background" Value="{StaticResource SMSBlue}"/>
        <Setter TargetName="Header" Property="Foreground" Value="White"/>
    </Trigger>
    <Trigger Property="TabIndex" Value="0">
        <Setter TargetName="Header" Property="Foreground" Value="Red"/>
    </Trigger>
</ControlTemplate.Triggers>

The window renders fine in the designer, but when I run the app:

System.InvalidOperationException: ''#FF008CFF' is not a valid value for property 'Background'.'

Note that my value for SMSBlue does not begin with #FF! And a Ctrl+Shift+F search finds that problem string nowhere in my project.

If I use the hex color directly, it runs fine:

<Setter TargetName="Panel" Property="Background" Value="#008CFF"/>

Runs fine even if I leave the resource color definition in my App.xaml.

Wha happened?

Perhaps the full exception details are helpful:

System.InvalidOperationException
  HResult=0x80131509
  Message='#FF008CFF' is not a valid value for property 'Background'.
  Source=WindowsBase
  StackTrace:
   at System.Windows.DependencyObject.GetEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, RequestFlags requests)
   at System.Windows.DependencyObject.GetValueEntry(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, RequestFlags requests)
   at System.Windows.DependencyObject.GetValue(DependencyProperty dp)
   at System.Windows.Controls.Panel.get_Background()
   at System.Windows.Controls.Panel.OnRender(DrawingContext dc)
   at System.Windows.UIElement.Arrange(Rect finalRect)
   at System.Windows.Controls.Border.ArrangeOverride(Size finalSize)
   at System.Windows.FrameworkElement.ArrangeCore(Rect finalRect)
   at System.Windows.UIElement.Arrange(Rect finalRect)
   at System.Windows.Controls.Control.ArrangeOverride(Size arrangeBounds)
   at System.Windows.FrameworkElement.ArrangeCore(Rect finalRect)
   at System.Windows.UIElement.Arrange(Rect finalRect)
   at System.Windows.Controls.Primitives.TabPanel.ArrangeVertical(Size arrangeSize)
   at System.Windows.Controls.Primitives.TabPanel.ArrangeOverride(Size arrangeSize)
   at System.Windows.FrameworkElement.ArrangeCore(Rect finalRect)
   at System.Windows.UIElement.Arrange(Rect finalRect)
   at System.Windows.Controls.Grid.ArrangeOverride(Size arrangeSize)
   at System.Windows.FrameworkElement.ArrangeCore(Rect finalRect)
   at System.Windows.UIElement.Arrange(Rect finalRect)
   at System.Windows.Controls.Grid.ArrangeOverride(Size arrangeSize)
   at System.Windows.FrameworkElement.ArrangeCore(Rect finalRect)
   at System.Windows.UIElement.Arrange(Rect finalRect)
   at System.Windows.Controls.Control.ArrangeOverride(Size arrangeBounds)
   at System.Windows.FrameworkElement.ArrangeCore(Rect finalRect)
   at System.Windows.UIElement.Arrange(Rect finalRect)
   at System.Windows.Controls.Grid.ArrangeOverride(Size arrangeSize)
   at System.Windows.FrameworkElement.ArrangeCore(Rect finalRect)
   at System.Windows.UIElement.Arrange(Rect finalRect)
   at MS.Internal.Helper.ArrangeElementWithSingleChild(UIElement element, Size arrangeSize)
   at System.Windows.Controls.ContentPresenter.ArrangeOverride(Size arrangeSize)
   at System.Windows.FrameworkElement.ArrangeCore(Rect finalRect)
   at System.Windows.UIElement.Arrange(Rect finalRect)
   at System.Windows.Controls.Decorator.ArrangeOverride(Size arrangeSize)
   at System.Windows.Documents.AdornerDecorator.ArrangeOverride(Size finalSize)
   at System.Windows.FrameworkElement.ArrangeCore(Rect finalRect)
   at System.Windows.UIElement.Arrange(Rect finalRect)
   at System.Windows.Controls.Border.ArrangeOverride(Size finalSize)
   at System.Windows.FrameworkElement.ArrangeCore(Rect finalRect)
   at System.Windows.UIElement.Arrange(Rect finalRect)
   at System.Windows.Window.ArrangeOverride(Size arrangeBounds)
   at System.Windows.FrameworkElement.ArrangeCore(Rect finalRect)
   at System.Windows.UIElement.Arrange(Rect finalRect)
   at System.Windows.Interop.HwndSource.SetLayoutSize()
   at System.Windows.Interop.HwndSource.set_RootVisualInternal(Visual value)
   at System.Windows.Window.SetRootVisual()
   at System.Windows.Window.SetRootVisualAndUpdateSTC()
   at System.Windows.Window.SetupInitialState(Double requestedTop, Double requestedLeft, Double requestedWidth, Double requestedHeight)
   at System.Windows.Window.CreateSourceWindow(Boolean duringShow)
   at System.Windows.Window.ShowHelper(Object booleanBox)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.DispatcherOperation.InvokeImpl()
   at MS.Internal.CulturePreservingExecutionContext.CallbackWrapper(Object obj)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at MS.Internal.CulturePreservingExecutionContext.Run(CulturePreservingExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Windows.Threading.DispatcherOperation.Invoke()
   at System.Windows.Threading.Dispatcher.ProcessQueue()
   at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
   at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
   at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
   at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
   at System.Windows.Application.RunDispatcher(Object ignore)
   at System.Windows.Application.RunInternal(Window window)
   at Gui.App.Main()

  This exception was originally thrown at this call stack:
    [External Code]

Try this instead:

<Application.Resources>
    <SolidColorBrush x:Key="SMSBlue" Color="#008CFF" />
</Application.Resources>

Or alternatively this:

<Application.Resources>
    <Color x:Key="SMSBlueColor">#008CFF<Color>
    <SolidColorBrush x:Key="SMSBlue" Color="{StaticResource SMSBlueColor}" />
</Application.Resources>