I was wondering this too
according to wikipedia....
The Steelers have used black and gold as their colors since the club's inception, the lone exception being the 1943 season when they merged with the Philadelphia Eagles and formed the "Steagles"; the team's colors at that time were green and white as a result of wearing Eagles uniforms. Originally, the team wore solid gold-colored helmets and black jerseys. Unique to Pittsburgh, the Steelers' black and gold colors are now shared by all major professional teams in the city, including the Pittsburgh Pirates in baseball and the Pittsburgh Penguins in hockey. These also are the colors of the city's official flag.
Stylized Logo
The Steelers logo was introduced in 1962 and is based on the "Steelmark," originally designed by Pittsburgh's U.S. Steel and now owned by the American Iron and Steel Institute (AISI). In fact, it was Cleveland-based Republic Steel that suggested the Steelers adopt the industry logo. It consists of the word "Steelers" surrounded by three astroids (hypocycloids of four cusps). The original meanings behind the astroids were, "Steel lightens your work, brightens your leisure, and widens your world." Later, the colors came to represent the ingredients used in the steel-making process: yellow for coal, orange for iron ore, and blue for scrap steel.[23] While the formal Steelmark logo contains only the word "Steel," the team was given permission to add "ers" in 1963 after a petition to AISI.
according to yahoo answers...
the pirates and steelers always had those colors...in the late 70s, the pirates won the world series and the steelers were in the superbowl. in honor of those teams, the penguins changed their colors to black and gold for the remainder of that season. but everyone liked it, so they stuck with the black and gold.
those sources are way shoddy but hey