Up here the state highway dept paints all it’s bridges green, a lightish green that is so common it must have a name, but I don’t know it. The railroad (B&M) painted all its bridges black. They still are black, although the B&M was bought out 20 years ago and hardly a wheel turns up here anymore, the bridges are still black, rather than rusty. Either the black was very long lasting, or the bridge fairy is still repainting them.
I use light green. It seems to be very common on plate girder bridges from the late 70s to today. At least out east.
Nick