A shared resource area (sulfur and iron) will not accept an iron mine. It will accept suflur. Iron mine stays yellow.
Not all new game starts have this issue, but many do. I can place the mine, (iron or copper) leave it yellow, save the game and reload and it will be operational. But the next mine I try to place in that lode it will do the same thing. Other ore lodes of those minerals act normal.
I see, I put it on my list.