Pepper-mint, not sure how long it would take in salt water to turn the wood grey. I'm more familiar with fresh water and it takes anywhere from a couple of months to several years, depending on the type of wood and how often it gets wet.
Salt water might speed up the process or it might slow it down, I don't know. And then you have the residual stain from the creosote-like preservative too, which would probably make it go grey faster.
Also, pine would turn grey faster than teak. And oak actually turns very black very quickly.
It's all relative.
My best guess would be that a pine deck would turn grey fairly quickly in worn spots, say a couple of months, but fresh wear would look light tan or whitish (the colour of bare pine), especially at the edges where it would wear quicker than in the center of a plank.
Keep an eye out for when you're travelling, and look at the fences in people's back yards, or at their decks. Notice how decks and verandas wear if they've been painted. Wooden buildings give you a lot of ideas, either bare wood like sheds (lighter near the top of a wall, darker near the bottom) and plank on frame houses show you how planks wear at the edges. It's all a matter of observation. (ok, I just finished watching Sherlock Holmes, but the same idea applies).
I sometimes test these things out by intentionally leaving a bit of wood outside and watching how it ages, or leaving a penny in tin full of water, sometimes salt, sometimes fresh, and watching tyhe colour changes. Looking at bronze statues (even buffing up a toe or something and then coming back in a few weeks to see how it's aged.)
