The red letter edition doesn't go back as far as you might think. According to the Wikipedia entry, a guy named Louis Klopsch came up with the idea. A fuller account of the event has been posted elsewhere, and it goes like this:

On June 19, 1899, the now Dr. Louis Klopsch was writing and editorial for the Christian Herald when his eyes fell upon Luke 22:20 and the words: "This cup is the new testament in my blood, which is shed for you." Dr. Klopsch realized that these were the words of our Saviour when he instituted the Lord's Supper. reasoning that all blood was red, he asked himself, "Why not a red letter Bible with the red words to be those of our Lord?" Dr. T. Dewitt Talmadge, pastor of the Brooklyn Temple where Louis and his father worshipped, encouraged him greatly by saying, "It could do no harm, and it most certainly could do much good."

Whether Talmadge was right or not is open to debate, but the fad certainly caught on. What's interesting to me, assuming this story isn't apocryphal, is how one man's rather eccentric and literal-minded notion came to be embraced to such an extent that there are people today who believe that not to print the words of Christ in red is somehow a sacrilege.

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Above: Red letter editions. Much good and no harm? You be the judge.

Compared to most books, the Bible features a text more footnoted and cross referenced and subdivided and interrupted than most, with an accretion of "helps" that assist (either greatly or not at all) with study at the risk of detracting from the reading experience. Whenever I hear people complaining about the difficulty of understanding what they've read in Scripture, I always wonder whether a de-numbered, de-referenced, uncluttered, uninterrupted experience of the text itself would have made a difference. 
Red letter editions seem to me to be an example of how misplaced piety can get in the way of what's really important -- in this case, a readable text. Don't get me wrong, compared to some of the other insertions, red ink is a minor distraction. Used tastefully for emphasis, the combination of black and red is a typographical classic, so I have no complaint on aesthetic grounds. But it's the sort of innovation I can really do without, because (a) it doesn't achieve a positive benefit and (b) in practical terms, it can go terribly wrong.
RED SCARE
Surely I'm wrong on my first point. Printing Christ's words in red ink is a pious and helpful thing to do, since it calls the reader's attention to the really important parts of the Bible. Well, yes and no. Depending on your view of inspiration, that dichotomy between the best bits and the rest can be truly unhelpful. If you believe that all Scripture is equally inspired, and that instead of opposing one passage to another, it's necessary to read them in harmony, then privileging the red letters above the rest is a tricky thing to do. You end up reinforcing the idea that the red sections teach something essentially different than all the others. 
Now, obviously, if you believe the words of Christ are more authoritative than the rest of the Bible, and even at odds with other parts, then a red letter edition makes perfect sense. But I don't think that's what Klopsch was getting at. It's just another example of how design choices can influence the way a text is read -- or not read -- or for that matter, red. 
PRACTICALLY SPEAKING
Part of my antipathy no doubt stems from the number of pink-letter and even orange-letter editions I've seen, the result of uneven printing. A nice dark crimson can look attractive on the page, but even at its best, four columns of tiny red print can be a bit of a strain. At its worst, it can be a disaster. These days, it's hard enough to get a nice dark imprint on decent paper. The nice thing about a black letter edition is, that's one less thing to get wrong.
THEN AGAIN . . .
Having said all this, I have to admit that, while it may shock the traditionalists among us, I can't help admiring the use of color in today's student Bibles. I'm not arguing that black text on white paper is the only acceptable option. What I am suggesting is that design choices be made with a certain end in mind, and where the Bible is concerned, that end ought to be a pleasant reading experience. Enhancements that contribute to this goal are worth exploring, while those that detract from it -- no matter how pious or well-intentioned -- ought to fade away.
I'm not against red letter editions, not with the same vehemence I have toward verse-per-line ones. But I do think it's worth questioning whether this is a tradition we ought to let go of. What do you think? Am I wrong here or what?

J. Mark Bertrand is a novelist and pastor whose writing on Bible design has helped spark a publishing revolution. Mark is the author of Rethinking Worldview: Learning to Think, Live, and Speak in This World (Crossway, 2007), as well as the novels Back on Murder, Pattern of Wounds, and Nothing to Hide—described as a “series worth getting attached to” (Christianity Today) by “a major crime fiction talent” (Weekly Standard) in the vein of Michael Connelly, Ian Rankin, and Henning Mankell.

Mark has a BA in English Literature from Union University, an MFA in Creative Writing from the University of Houston, and an M.Div. from Heidelberg Theological Seminary. Through his influential Bible Design Blog, Mark has championed a new generation of readable Bibles. He is a founding member of the steering committee of the Society of Bible Craftsmanship, and chairs the Society’s Award Committee. His work was featured in the November 2021 issue of FaithLife’s Bible Study Magazine.

Mark also serves on the board of Worldview Academy, where he has been a member of the faculty of theology since 2003. Since 2017, he has been an ordained teaching elder in the Presbyterian Church in America. He and his wife Laurie life in Sioux Falls, South Dakota.