VisualTreeHelper を介して ColorPicker からポップアップを取得し、境界線のプロパティ (ポップアップの子) を次のように変更します。
private void colorPicker_Loaded(object sender,RoutedEventArgs e)
{
Popup popup = FindVisualChildByName<Popup> ((sender as DependencyObject),"PART_ColorPickerPalettePopup");
Border border = FindVisualChildByName<Border> (popup.Child,"DropDownBorder");
border.Background = Brushes.Yellow;
}
private T FindVisualChildByName<T>(DependencyObject parent,string name) where T:DependencyObject
{
for (int i = 0;i < VisualTreeHelper.GetChildrenCount (parent);i++)
{
var child = VisualTreeHelper.GetChild (parent,i);
string controlName = child.GetValue (Control.NameProperty) as string;
if (controlName == name)
{
return child as T;
}
else
{
T result = FindVisualChildByName<T> (child,name);
if (result != null)
return result;
}
}
return null;
}