Those are all headshots, are you sure aren't referring to their headgear, rather than the totality of their outfit?

In any case, the variations of close-fitting cap they're all wearing is generally referred to as a coif; but I don't think that's very interesting. What I do think is interesting is the fact that they're all wearing laurel wreaths! I actually wrote about this tradition in the past; what fundamentally happened was that the Ancient Roman-style laurel-wreath coronation was revived by the fourteenth-century Papacy as a ceremony celebrating popular intellectuals. That's incidentally where the modern title "Poet Laureate" comes from.