public Brush ShapeColor{ get; set; }

public Color StoreColor
    {
        get
        {
             g = ((Color)ShapeColor.GetValue(SolidColorBrush.ColorProperty)).G;
             r = ((Color)ShapeColor.GetValue(SolidColorBrush.ColorProperty)).R;
             b = ((Color)ShapeColor.GetValue(SolidColorBrush.ColorProperty)).B;
             return Color.FromRgb(r, g, b);
        }
        set {}
    }

The calling thread cannot access this object because a different thread owns it.

g = ((Color)ShapeColor.GetValue(SolidColorBrush.ColorProperty)).G;
r = ((Color)ShapeColor.GetValue(SolidColorBrush.ColorProperty)).R;
b = ((Color)ShapeColor.GetValue(SolidColorBrush.ColorProperty)).B;