ActivityIndicator Color on PullToRefresh in ListView

I'm using PullToRefresh in my ListView. When the ItemsSource of my ListView is empty the displayed ActivityIndicator on a PullToRefresh operation is black, when the ItemsSource has items in it the color of the ActivityIndicator is white. Does Xamarin calculate the color based on the color used in the cells? Why does it work like this?

I would like to have it white at all times and i tried several things like setting the overall appearance of the UIActivityIndicator with the following lines:

UIActivityIndicatorView.Appearance.Color = UIColor.White;

Somehow that isn't working either. Any ideas on how to get the required result?

Thanks a lot.