"A heading, lemma, title, or initial letter written in red as a means of highlighting it, from Latin ruber ("red") and rubrico ("to colour red"). In post post-Augustan Rome, the term rubrica came to be applied to the individual titles or laws in Roman law, which were picked out in red in the great codifications.
It was this practice which was adopted in medieval manuscript production as a means of signalling the sections in other sorts of work as well. One of the earliest appearances in Christian texts was in liturgical books, where the directions for the conduct of the mass were rubricated."
From T.S. Haskett's Some Notes on Palaeography, Codicology and Diplomatics, 2001