(Mostly a creation of the Hyper Colliseum card game)
The Seven Demon Lords are evil Digimon each connected to one of the Seven Deadly Sins, a color, an ancient planet, and a member of the "real" Demonic host:
Lucemon is associated with Lucifer, the quintessential Fallen Angel, and the sin of Pride. His planet is the Sun, and his color is red.
Beelzemon is associated with Beelzebub, the Lord of the Flies, and with the sin of Gluttony--meaning, in his case, power more than food. His ancient planet is Venus, and his color is yellow.
Daemon is associated with Satan, and the sin of Wrath. His planet is Jupiter, and his color is orange.
Belphemon is associated with Belphegor and the sin of Sloth--he enters a Sleep Mode and stays there for one thousand years (and when he wakes up he's pissed). His planet is Mars, and his color is blue.
Leviamon is associated with the Leviathan, and the sin of Envy--it is said that, like Cherubimon, he feels envious of his more "human type" fellow Demon Lords. His planet is Mercury, and his color is light blue.
Barbamon is associated with Mammon, and the sin of Greed. His planet is Saturn, and his color is violet.
Lilithmon is associated with Lilith, the first wife of Adam from the Hebrew Bible, who rejected him and bore demon offspring, and the sin of Lust. Her planet is Luna, the Moon, and her color is green.
[list courtesy of Hikari no Hohenheim from wtw and guilmon93950 from Digimonccg.com]