Translation:The tree at the beach is not green.
i don't think the English on vs. at captures this at all. To my ears, "the tree at the beach" sounds unnatural and means pretty much the same thing as "the tree on the beach". For what it's worth, in English people say they have a house on the beach. They don't mean the house is literally in the intertidal zone. They mean they have a house right by the beach. So I'm not sure the op/aan distinction can be neatly mapped to on/at.
I was inclined to disagree at first, but I think you're right, Nideva. "On the beach" and "At the beach" do seem to be good translations for "op de strand" and "aan de strand," respectively. They're probably not exact equivalents in meaning, because Dutch and English are simply different languages, but I think using "on" versus "at" in English helps draw a useful distinction between the two.
It's true that when we talk about a house being "on the beach" we mean "overlooking it," but I think this is specific to buildings: we say a house is "on the square," where I believe Dutch uses "bij." If we're talking about a smaller object being on the beach, I think that pretty much places it in the sand (or pebbles--whatever the beach is made out of.) Whereas "at the beach" means more broadly "in the vicinity of the beach." I don't think it's a direct equivalent for "aan," here--"by" would be better--but at least it allows the tree to be near the beach without having to be in the sand.