Just to clarify, pure zinc chromate is a really sick looking yellowish green...most of what we know as zinc Chromate primer has had colorants added...and yes, it darkens over time so when looking at older items it would appear as dark green.
I would hope any restoration needing Zinc Chromate as a color would use a modern paint that approximated the original color...I suppose the stuff must be available from some chemical supply, but true Zinc Chromate is not only toxic it is a carcinogen.
I can't think of any historic railroad restoration project need today for the properties of Zinc Chromate that would be worth the risk.