The address with the letter is written with emerald green ink with the stamp of Hogwarts School of Witchcraft and Wizardry placed on it. The letter magically knows where the recipient is living at the time and if they move.
The letter is written on parchment paper and delivered to the recipient by owl, except in the case of Muggle-borns and magical children living with Muggles (such as Harry Potter), where the initial acceptance letter is delivered by the regular Muggle post.
We are pleased to inform you that you have been accepted at Hogwarts School of Witchcraft and Wizardry. Please find enclosed a list of all necessary books and equipment. Term begins on 1 September. We await your owl by no later than 31 July.
Emerald green. Right there in chapter 3.
The Hogwarts™ Express, with its iconic scarlet steam engine, symbolizes the magical journey that awaits students at Hogwarts™ school of witchcraft and wizardry.
There is only one way that a student is selected to go to Hogwarts, and that choice isn't made by a person. In fact, it is made by a book and quill. In a tower at Hogwarts, where no student goes, sits the Book of Admittance and Quill of Acceptance. These objects decide who gets a place at Hogwarts.
When a student was Muggle-born the acceptance letter was delivered in person by a member of the Hogwarts faculty. The faculty member acted as a special messenger, who explained everything to the student and parents.
Yet here it was, a letter, addressed so plainly there could be no mistake: The envelope was thick and heavy, made of yellowish parchment, and the address was written in emerald-green ink.