When you see your tefillin paint fading, bring them in. Is it halachah or aesthetics? Although b’dieved if tefillin are not black they may still be used, there are those (reshonim) who hold that black battim is a halacha l’moshe m’sini (a law that was passed down orally from the times of Moshe), namely Tosafos, Mordechai, Noda B’Yehuda, Vilna Gaon. If this would be your Shabbos suit would you wear it? The Shabbos Suit Testis my way of making people aware of the appearance of their tefillin. We all understand that it would be incongruous to be makpid (careful) on every detail of our dress on the one hand, and uncaring as to the appearance of the object with which we fulfill one of our most precious mitzvos. Our crown, our jewel, that’s what we don’t have time to keep in perfect looking shape? what we are going to So be consistent and get your holy tefillin painted.