Exodus 28:36-38

36“Also you are to make a plate of pure gold and engrave on it, like the engravings of a seal, ‘HOLY TO Adonai.’

37Attach it with a violet cord onto the turban, on the front of the turban.

38So it will rest on Aaron’s forehead, so that Aaron will bear away the iniquity committed regarding the holy things, which Bnei-Yisrael set apart as all their holy gifts. It is to be always on his forehead, so that they may have favor before Adonai.