I have been studying the book by Joseph Sheppard titled 'How to Paint Like the Old Masters' and I have noticed that Rubens and Titian are said to have used ivory black and white to paint in shadows after the burnt umber underpainting. I have been reading a lot about underpainting on this site and haven't come across this step yet. Yet another confusing step!

Any clarification by anyone who has had experience with this would be greatly appreciated!
Thanks,
Joan