Rubricating has been around for ages. It must have been tedious for scribes working only in black ink after a while, and what better way to highlight something than to change its colour. But did it have to be red? Was there not a less laborious colour to mix? This is how red ink was made at the time:

To prepare white-flake, get some sheets of lead beaten out thin, place them dry in a hollow piece of wood and pour in some warm vinegar or urine to cover them. Then, after a month, take off the cover and remove whatever white there is, and again replace it as at first. When you have a sufficient amount and you wish to make red lead from it, grind this flake-white on a stone without water, then put it in two or three new pots and place it over a burning fire. You have a slender curved iron rod, fitted at one end in a wooden handle and broad at the top, and with this you can stir and mix this flake-white from time to time. You do this for a long time until the red lead becomes visible. (from Theophilus' De diversis artibus).

But it would have been worth the effort considering the hours spent on the black parts of the book or manuscript.

I used a powerful computer and a pencil to mix mine. Took about 15 seconds.