On one side is the Prado, a renowned repository of art and a showcase of Spanish culture that draws huge numbers of tourists. On the other is a brash newcomer, emerging onto the scene in layers of gray granite from a hillside near the baroque royal palace of Spanish kings.

In advance of its opening, the upstart, the Museum of Royal Collections, is insisting that the Prado surrender four paintings, including its top two attractions — “The Garden of Earthly Delights” by Hieronymus Bosch and a sumptuous 15th-century depiction of the descent of Christ from the cross by Rogier van der Weyden.