My rule is, when deciding to paint or not, is to stain first. If you don’t like the way it looks, then consider painting. There will be no paint!
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The deck will become darker after stain and finishing. But I do want a contrast between the deck and walls.

Now I have to start on the other walls, which are at angles. This will help me determine where to stop the deck boards, as it’s looking like I could run out with three more decks to go. But I can always buy more wood!
So the goal now is to build the walls, and that will allow me to finish planking and add nails. Then the walls will go in permanently so the upper decks can go on top of them.