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Coal and iron ore. That's the black and gold behind the city of Pittsburgh.
The Steelers aren't the only team to incorporate the culture of the city in which it plays into their brand, but they are the only one to incorporate the city into its uniforms.
NFL Films did a great, albeit short, piece on the history of the Steelers' colors, and included in the two minute clip are interviews with uniform historian Scott Sillcox and Pro Football Hall of Fame's Joe Horrigan. Each of them comment specifically on the Steelers' colors, and the video gives an overview on how they came to be.
There's some great old footage in here depicting Pittsburgh in the 1970s, including clips of Dan Rooney as a much younger man, commenting on how Pittsburgh's Golden Triangle was incorporated into the jerseys, but wasn't recognized.
It's very interesting, and it could have gone on for much longer than just two minutes, but it's worth the watch. Especially for the historian's perspective on the jerseys the Steelers chose as throwbacks for this season.