Symbolism, Imagery, Allegory
When the Kingdom of the Two Sicilies joined the United Kingdom of Italy, they went from having this flag to having this one.
And if that weren't enough of a historical tidbit, the three-color design of the later flag was modeled on the flag France adopted after their revolution in 1789. Prince Fabrizio sees this design for the first time at a ball in Donnafugata. But instead of a flag, the party hosts show this symbol using glasses of colored wine:
On a small table was a plate with some ancient biscuits covered with fly droppings and a dozen little squat glasses brimming with rosolio: four red, four green, four white, the last in the center: an ingenuous symbol of the new national flag which tempered the Prince's remorse with a smile. (3.29)
Fabrizio smiles at this because the spread of drinks and biscuits is already covered in fly droppings. He knows that one day, the Kingdom of Italy won't exist anymore, just like his own Kingdom of the Two Sicilies no longer exists.
History would go on to prove Fabrizio right, too, as the Kingdom of Italy would stop existing in 1946. Lampedusa knew this when he wrote The Leopard, so having the fly-poop speckled tablecloth is extra poignant. And it's extra gross, too, even without the symbolism. Yuck.