Black, firered, diamond, platinum, emerald, yellow, are all examples.
Because it is just Nintendo's style to have a common theme when naming a group of things.
For example, all cities in Kanto are named after colors (Viridian City, Lavender Town, Indigo Plateau), the cities in Hoenn are two words put together (Little Root, Fort Tree, Slate Port, Ever Grande, etc.) and other things.
EDIT: Also Johto areas named from flowers/plants/trees (Cherrygrove, Blackthorn, Goldenrod etc) and Unova cities named after clouds (thx DT and speed freak)