Red was always the secondary ink color, used for heading, running titles, initials, rubrics, & red-letter days. It was sometimes employed to mark mistakes. Use of red ink goes back to at least the 5th Century and flourished till the 15th. Printing killed the use of red (or any other colors for words).
Sometimes blue and green ink would be used as a third and fourth color, but they are not nearly as common. (Scribes & Illuminators, 1992, p. 33)