As always happens when I travel, something will catch my brain and I won’t be able to let it go until I find the answer.
I have been to Pittsburgh 4 times in my life and the last 2 I have wondered about the yellow bridges. In Harrisburg most of our painted bridges are blue. That is a color that shows rust easily so they can be repaired quickly (or so my roller coaster building friends have said.. think Skyrush at Hersheypark.)
Turns out Pittsburgh bridges are not yellow, they are Aztec Gold. And it is because they connect to the Golden Triangle. Yep, I googled it. Here’s a link to one of the articles I liked about it.