WPF pen color problem in OnRender of Custom Adorner

public class ResizeableSelectionAdorner : Adorner
    {
        Thumb topLeft, topRight, bottomLeft, bottomRight;
        DrawingSizeIndicator drawingSizeIndicator;
        VisualCollection visualChildren;

        public ResizeableSelectionAdorner(UIElement adornedElement)
            : base(adornedElement)
        {
            visualChildren = new VisualCollection(this);

            // Call a helper method to initialize the Thumbs
            // with a customized cursors.
            BuildAdornerCorner(ref topLeft, Cursors.SizeNWSE);
            BuildAdornerCorner(ref topRight, Cursors.SizeNESW);
            BuildAdornerCorner(ref bottomLeft, Cursors.SizeNESW);
            BuildAdornerCorner(ref bottomRight, Cursors.SizeNWSE);

            bottomLeft.DragDelta += new DragDeltaEventHandler(HandleBottomLeft);
            bottomRight.DragDelta += new DragDeltaEventHandler(HandleBottomRight);
            topLeft.DragDelta += new DragDeltaEventHandler(HandleTopLeft);
            topRight.DragDelta += new DragDeltaEventHandler(HandleTopRight);

            bottomLeft.GotMouseCapture += new MouseEventHandler(Corner_GotMouseCapture);
            bottomRight.GotMouseCapture += new MouseEventHandler(Corner_GotMouseCapture);
            topLeft.GotMouseCapture += new MouseEventHandler(Corner_GotMouseCapture);
            topRight.GotMouseCapture += new MouseEventHandler(Corner_GotMouseCapture);

            bottomLeft.LostMouseCapture += new MouseEventHandler(Corner_LostMouseCapture);
            bottomRight.LostMouseCapture += new MouseEventHandler(Corner_LostMouseCapture);
            topLeft.LostMouseCapture += new MouseEventHandler(Corner_LostMouseCapture);
            topRight.LostMouseCapture += new MouseEventHandler(Corner_LostMouseCapture);

            drawingSizeIndicator = new DrawingSizeIndicator();
            drawingSizeIndicator.Opacity = 0;
            visualChildren.Add(drawingSizeIndicator);
        }

        void Corner_LostMouseCapture(object sender, MouseEventArgs e)
        {
            bottomLeft.Opacity = 0.4;
            bottomRight.Opacity = 0.4;
            topLeft.Opacity = 0.4;
            topRight.Opacity = 0.4;
            drawingSizeIndicator.Opacity = 0;
            InvalidateVisual();
        }

        void Corner_GotMouseCapture(object sender, MouseEventArgs e)
        {
            bottomLeft.Opacity = 0;
            bottomRight.Opacity = 0;
            topLeft.Opacity = 0;
            topRight.Opacity = 0;
            drawingSizeIndicator.Opacity = 1;
            InvalidateVisual();
        }

        void HandleBottomRight(object sender, DragDeltaEventArgs args)
        {
            FrameworkElement adornedElement = this.AdornedElement as FrameworkElement;
            Thumb hitThumb = sender as Thumb;

            if (adornedElement == null || hitThumb == null) return;
            FrameworkElement parentElement = adornedElement.Parent as FrameworkElement;

            // Ensure that the Width and Height are properly initialized after the resize.
            EnforceSize(adornedElement);

            // Change the size by the amount the user drags the mouse, as long as it's larger 
            // than the width or height of an adorner, respectively.
            adornedElement.Width = Math.Max(adornedElement.Width + args.HorizontalChange, hitThumb.DesiredSize.Width);
            adornedElement.Height = Math.Max(args.VerticalChange + adornedElement.Height, hitThumb.DesiredSize.Height);
        }

        void HandleBottomLeft(object sender, DragDeltaEventArgs args)
        {
            FrameworkElement adornedElement = AdornedElement as FrameworkElement;
            Thumb hitThumb = sender as Thumb;

            if (adornedElement == null || hitThumb == null) return;

            // Ensure that the Width and Height are properly initialized after the resize.
            EnforceSize(adornedElement);

            // Change the size by the amount the user drags the mouse, as long as it's larger 
            // than the width or height of an adorner, respectively.
            if (adornedElement.Width != Math.Max(adornedElement.Width - args.HorizontalChange, hitThumb.DesiredSize.Width))
            {
                double x = Canvas.GetLeft(adornedElement) + args.HorizontalChange;
                Canvas.SetLeft(adornedElement, x);
                adornedElement.Width = Math.Max(adornedElement.Width - args.HorizontalChange, hitThumb.DesiredSize.Width);
            }
            adornedElement.Height = Math.Max(args.VerticalChange + adornedElement.Height, hitThumb.DesiredSize.Height);
        }

        void HandleTopRight(object sender, DragDeltaEventArgs args)
        {
            FrameworkElement adornedElement = this.AdornedElement as FrameworkElement;
            Thumb hitThumb = sender as Thumb;

            if (adornedElement == null || hitThumb == null) return;
            FrameworkElement parentElement = adornedElement.Parent as FrameworkElement;

            // Ensure that the Width and Height are properly initialized after the resize.
            EnforceSize(adornedElement);

            // Change the size by the amount the user drags the mouse, as long as it's larger 
            // than the width or height of an adorner, respectively.
            adornedElement.Width = Math.Max(adornedElement.Width + args.HorizontalChange, hitThumb.DesiredSize.Width);
            double y = Canvas.GetTop(adornedElement) + args.VerticalChange;
            Canvas.SetTop(adornedElement, y);
            adornedElement.Height = Math.Max(adornedElement.Height - args.VerticalChange, hitThumb.DesiredSize.Height);
        }

        void HandleTopLeft(object sender, DragDeltaEventArgs args)
        {
            FrameworkElement adornedElement = AdornedElement as FrameworkElement;
            Thumb hitThumb = sender as Thumb;

            if (adornedElement == null || hitThumb == null) return;

            // Ensure that the Width and Height are properly initialized after the resize.
            EnforceSize(adornedElement);

            // Change the size by the amount the user drags the mouse, as long as it's larger 
            // than the width or height of an adorner, respectively.
            if (adornedElement.Width != Math.Max(adornedElement.Width - args.HorizontalChange, hitThumb.DesiredSize.Width))
            {
                double x = Canvas.GetLeft(adornedElement) + args.HorizontalChange;
                Canvas.SetLeft(adornedElement, x);
                adornedElement.Width = Math.Max(adornedElement.Width - args.HorizontalChange, hitThumb.DesiredSize.Width);
            }

            if (adornedElement.Height != Math.Max(adornedElement.Height - args.VerticalChange, hitThumb.DesiredSize.Height))
            {
                double y = Canvas.GetTop(adornedElement) + args.VerticalChange;
                Canvas.SetTop(adornedElement, y);
                adornedElement.Height = Math.Max(adornedElement.Height - args.VerticalChange, hitThumb.DesiredSize.Height);
            }
        }

        protected override Size ArrangeOverride(Size finalSize)
        {
            // desiredWidth and desiredHeight are the width and height of the element that's being adorned.  
            // These will be used to place the ResizingAdorner at the corners of the adorned element.  
            double desiredWidth = ActualWidth;
            double desiredHeight = ActualHeight;
            // adornerWidth & adornerHeight are used for placement as well.
            double adornerWidth = this.DesiredSize.Width;
            double adornerHeight = this.DesiredSize.Height;

            topLeft.Arrange(new Rect(-adornerWidth / 2, -adornerHeight / 2, adornerWidth, adornerHeight));
            topRight.Arrange(new Rect(desiredWidth - adornerWidth / 2, -adornerHeight / 2, adornerWidth, adornerHeight));
            bottomLeft.Arrange(new Rect(-adornerWidth / 2, desiredHeight - adornerHeight / 2, adornerWidth, adornerHeight));
            bottomRight.Arrange(new Rect(desiredWidth - adornerWidth / 2, desiredHeight - adornerHeight / 2, adornerWidth, adornerHeight));

            drawingSizeIndicator.Arrange(new Rect(-drawingSizeIndicator.ElementIndicatorLength+1,-drawingSizeIndicator.ElementIndicatorLength,
                desiredWidth + drawingSizeIndicator.ElementIndicatorLength * 4 + drawingSizeIndicator.SizeIndicatorLength-2,
                desiredHeight + drawingSizeIndicator.ElementIndicatorLength * 4 + drawingSizeIndicator.SizeIndicatorLength-1));

            // Return the final size.
            return finalSize;
        }

        // Helper method to instantiate the corner Thumbs, set the Cursor property, 
        // set some appearance properties, and add the elements to the visual tree.
        void BuildAdornerCorner(ref Thumb cornerThumb, Cursor customizedCursor)
        {
            if (cornerThumb != null) return;

            cornerThumb = new Thumb();

            // Set some arbitrary visual characteristics.
            cornerThumb.Cursor = customizedCursor;
            cornerThumb.Height = cornerThumb.Width = 10;
            cornerThumb.Opacity = 0.40;
            cornerThumb.Background = new SolidColorBrush(Colors.MediumBlue);

            visualChildren.Add(cornerThumb);
        }

        void EnforceSize(FrameworkElement adornedElement)
        {
            if (adornedElement.Width.Equals(Double.NaN))
                adornedElement.Width = adornedElement.DesiredSize.Width;
            if (adornedElement.Height.Equals(Double.NaN))
                adornedElement.Height = adornedElement.DesiredSize.Height;

            FrameworkElement parent = adornedElement.Parent as FrameworkElement;
            if (parent != null)
            {
                adornedElement.MaxHeight = parent.ActualHeight;
                adornedElement.MaxWidth = parent.ActualWidth;
            }
        }

        protected override int VisualChildrenCount { get { return visualChildren.Count; } }
        protected override Visual GetVisualChild(int index) { return visualChildren[index]; }
    }