E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
(from the Latin rubrca, red ochre, or vermilion). An ordinance or law was by the Romans called a rubric, because it was written with vermilion, in contradistinction to prætorian edicts or rules of the court, which were posted on a white ground. (Juvenal, xiv. 192.)
Rubrca vetvit = the law has forbidden it. (Persius, v. 99.)
Prætres edicta sua in albo proponebant, ac rubrcas [i.e. jus civile] translalrunt.Quintilian, xii. 3, 11.
Rules and orders directing how, when, and where all things in divine service are to be performed were formerly printed in red characters (now generally in italics), and called rubrics.Hook: Church Dictionary.