Must tefillin be written with black ink, and does writing even one letter in another color or in gold invalidate the tefillin?

Synopsis

All authorities agree tefillin must be written in black ink. Writing even one letter in another color or in gold invalidates the tefillin entirely. The Rema adds a l'chatchila preference for ink made from smoked wood or oil soaked in gallnut juice.

Early Acharonim
Lenient
Strict
Lesser cited
Major authority

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