Is this Mold or Discoloration? Or Rooted Wood attic...

So, I’m confused, the wood in the attic has black streaks/discolorations. Rusty nails. I thought this was a ventilation problem, someone came to look at the wood and said it was fine.

But, then I read that this could be mold? There are a couple of boards in the pictures that I circled that are a grayish color. Is that rotted wood? I attached three pictures.

The first and second pictures are “No”. The third might be “Yes” or it might be the way the picture was taken that gives an illusion. Does it wipe off when you touch it?

Looks like they have been replaced at some point in time (top boards). If it’s rotten or decayed you should be able to tell with an awl or screwdriver.
Others look like moisture stains from the frosted nails in winter or wood shakes wet clean through.

No mold or rot. From my perspective.May have been live mold at sometime.
Looking at the lumber and the roofing nail spacing. I think the structure is from the sixties or older…Thinking a lot older.
So! That being said, There was significant amount of humidity in times past to cause this. It is dead now…Just my thoughts.
I’d have no problem with it.

Possibly when the house was built they ran out of boards and used three concrete form boards to complete the roof deck. Just a thought.

I agree with Kenneth, frost and/or condensation on the nails likely contributed to the dark stains. Also it was common practice before plywood to reuse the form boards as sub-flooring or roof sheathing.