Aptly, as the snow around the convent begins to melt we proudly present to you, a cardinal's "winter" biretta. It is made of fine wool serge. Today this is the only biretta a cardinal may wear outside of liturgical functions, as when he is in choir dress, he wears the watered silk biretta instead. In abito piano it must be the wool version, as displayed here.
Most cardinals these days wear only the watered silk scarlet pileolus outside liturgical functions, which as you might expect is utterly beyond our comprehension. Really, can you imagine being allowed to wear a hat and then choosing not to?