Is the powerwashing the subcontractor did on our deck "good enough?"
I think it looks pretty good. If something was clearly missed you could call them back for those area. I once used a solid color stain on my deck, but I didn't like that once it peeled, it looked bad. I recommend a semi-transparent stain. The gray and black areas will add character. Try a small area and see how it looks before doing the whole deck. I would never paint it or even worse, use one of those "deck restore" products that goes on super thick. Once that stuff starts peeling, (and it will), you would have a huge mess on your hands. Fortunately, all you need is some stain with a good protectant in it.