The accent on the 'a' of están is correct.
The sentence you wrote has a misspelling, however, and should read:
Las manzanas están verdes.
This sentence means that the apples are green at the present time (they are unripe, not ready to eat).
Las manzanas son verdes.
This sentence describes apples that are characteristically green, like Granny Smith apples, when ripe (ready to eat).
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