Watch my hands.
K might turn out to be the trickiest letter in the color alphabet. Most lists had exactly one entry for it: Khaki, which is not only a "meh" color, it also has no good stories attached to it. So I decided to go the alternate route and look at colors in Hungarian (hey it's not cheating if you're bilingual!). This is how I ran into cadmium colors, and from there it was only a step further to spell them the Greek way.
In turn, cadmean earth, first found near Thebes, was named after the city's founder and first king, Cadmus, or rather, Kadmos (Greek spelling).
Kadmos was not Greek. He was Phoenician, and also happened to be the brother of Europe, the girl that was kidnapped by a very horny Zeus and gave her name to a continent. While being the mother of Zeus' children is usually presented as an honor in Greek mythology, parents rarely ever approve. In this case, they sent big brother Kadmos to bring Europe back home.
In the process, however, he managed to piss off Ares, god of war, who happened to be fond of that dragon in particular. Kadmos ended up serving him for several years before he was granted forgiveness, and Ares and Aphrodite's daughter, Harmonia. But even then, misfortune followed him till the end of his life, until he cursed the entire dragon thing over and over. The gods, who have a wicked sense of humor, turned him (and his wife) into dragons.
Kadmos is obviously tied to the earth with many threads in these stories (building a city, growing warriors from the ground, slaying a dragon and then becoming one). No wonder they credit him with cadmean earth (and also minding gold for his kingdom). Incidentally, they also credit him with bringing the Phoenician alphabet to Hellas.