Why are some cardinals wearing black?

Eagle-eyed viewers may notice something peculiar as the members of the College of Cardinals arrive in Rome ahead of Pope Francis' funeral and the upcoming conclave: some of them seemed to have missed the fashion memo.

In the sea of red birettas, the men clad in black vestments and hats stand out. But what's going on?

When in public or at a diplomatic event, the cardinals wear "choir dress" — the red outfit one imagines when they hear the word "cardinal." That includes conclaves.

(Fun fact: Cardinals, the birds, were named after cardinals, the people, because their red feathers were reminiscent of the cardinals' vestments.)

But that only applies to cardinals who are members of the Latin Church. Which, to be fair, is most of them.

The Catholic Church is more than just one Church — it's 24 Churches. While the vast majority of the world's Catholics are "Latin Rite" Catholics, about 18 million of them belong to one of the 23 "sui iuris" Churches.

The largest of these churches are the Ukrainian Greek Catholic Church and the Syro-Malabar Church, each with over 4 million members. The smallest Churches have only a few thousand members.

By comparison, the Latin Church has over a billion members.

With the exception of the Maronite Catholic Church, these churches were all formerly Eastern or Oriental Orthodox and elected to return to full communion with Rome.

While these Churches are autonomous, they are in full communion with Rome and acknowledge the pope as their leader, but still maintain some of their own liturgical traditions and customs.

Case in point: liturgical dress.

Those men wearing black? They're cardinals of Eastern Churches.

Eastern Catholic bishops who are elevated to the College of Cardinals typically keep their traditional attire. So no red.

There are presently five cardinal electors who belong to Eastern Catholic Churches.

These are Cardinal Louis Raphael I Sako, a Chaldean Catholic; Cardinal Baselios Cleemis of the Syro-Malankara Church; Cardinal Berhaneyesus Demerew Souraphiel, an Ethiopian Catholic; Cardinal Mykola Bychok of the Ukrainian Greek Catholic Church; and Cardinal George Koovakad of the Syro-Malabar Church.

One of those has an interesting distinction: Cardinal Mykola Bychok of the Ukrainian Greek Catholic Church who is serving in Australia is the youngest cardinal.

The other three cardinals, Cardinal Bechara Boutros al-Rahi of the Maronite Catholic Church; Cardinal George Alencherry of the Syro-Malabar Catholic Church; and Cardinal Lucian Muresan of the Romanian Greek Catholic Church, at ages 85, 80, and 93, respectively, are too old to vote in the conclave. Cardinal Alencherry only missed the chance by a couple days; his 80th birthday was April 19.