Many New Testaments have Words in Red to distinguish when Jesus is speaking. These are referred to as Red Letter Edition Bibles, or simply described as Words of Christ in Red. Wherever it is possible the Red Letters are used to help the reader discern the actual statements or conversations which can be directly attributed to Jesus. The object is to make it clear when Jesus is being quoted, as opposed to someone speaking to or about Him. It is not the publisher's intent to somehow indicate that the red letters are more important, it is simply a reading study tool.
Matthew, Mark, Luke and John are the Disciples who wrote the accounts of Jesus' time on earth, so the majority of His spoken dialogue is in those books. Some other New Testament books may have references to something He said, but those verses may not appear in red because they are not direct quotes. For example, it is true Romans does not contain the words Jesus spoke during his earthly ministry. It could be argued that what Paul wrote could be considered the "words of Christ" because as an apostle he would be writing what he learned from Jesus. Most people would still prefer to have only the Words in Red when Jesus is talking; Parables, stories, prayers, and other direct quotes.
While He was on earth Jesus told the apostles He had many things to tell them, but they were not able to bear it then. (John 16:12) Jesus began teaching about salvation but wanted them to preach the Gospel after He ascended back into Heaven. (Heb. 2:3,4; 1 Thess. 2:4) The Lord promised them that when He left, the Spirit would come and would be constantly reminding them of everything Jesus had said, and that the Holy Spirit would guide them into all truth. (John 14:26; 16:13) They would be proclaiming God's Word as Jesus said, "Whoever hears what you say, is hearing it just as if I were saying it; so whoever rejects you and your message, is really rejecting Me; and because that is true, whoever rejects Me is also rejecting My Father Who Me." (Luke 10:16)
The apostles did not compose their own teachings completely separate from what Christ had taught them. Rather, they all constantly referred back to all the words of Christ, because they were guided by the same Holy Spirit. Though Paul became an apostle after the other disciples, he wrote that his preaching was in total agreement with the original apostles. (Gal. 2:1-9) Paul spoke and wrote by inspiration just as the other apostles did. As a matter of fact many Christians believe Paul actually physically spent time with the resurrected Christ and received much of the doctrine he wrote about from that time with the Lord. So it is no surprise he told the Corinthians,
"...the things which I write to you are the commandments of the Lord..." (1 Cor. 14:37)
The books of Acts through Revelation actually complete much of Christ's teaching. These books contain things the Jesus referred to when He told the apostles they could not bear to hear it before His death. The Apostle John wrote,
"And there are also many other things that Jesus did, which if they were written one by one, I suppose that even the world itself could not contain the books that would be written." (John 21:25)
Obviously if that statement is true, it would also mean there are many other things Jesus said, as well as did. So the idea that the words of Christ in red, could be the final statements Jesus made is not possible. Which makes more evident the truth that what the other Apostles (meaning Peter, James and Paul) wrote, also contains some of Jesus' words, simply not printed in red. Acting as his ambassadors they conveyed the words of their King to the world in due time. (2 Cor. 5:20; 1 Tim. 2:6) So you could say the words of Christ are in red AND black.
There is no right or wrong about red letter Bibles, so long as we remember the words in black are just as important as those in red. There is no contradiction between what Jesus personally said and what Peter, James, John or Paul recorded. You could say we should study the Gospel according to Matthew, Mark, Luke, John and anyone else who quoted what they personally heard Jesus say.
The above article is in part to be referenced to Doug Hoff, The Bible.net
used by permission.
Origin of Red-Letter Bibles
Red-letter Bibles have become so common that it's easy to assume they have been around for as long as Bibles have been printed. Not so! The first red-letter New Testament was published in 1899, and the first red-letter Bible followed two years later.
The idea of printing the words of Christ in red originated with Lous Klopsch, editor of Christian Herald magazine. Klopsch was a close friend of such contemporaries as T. DeWitt Talmage, and world-renowned preacher D.L. Moody, and well-known hymnist Ira Sankey. Klopsch was an early supporter of Moody's Bible Institute in Chicago, rallying Christian Herald readers to send in contributions for the financially strapped school. In his eighteen years as proprietor of Christian Herald, Klopsch raised more than three billion dollars for relief work throughout the world. 
No cause was dearer to Klopsch's heart, however, than that of Scripture distribution and reading. Through Christian Herald Klopsch published more than 60,000 Bibles and Testaments annually during much of his tenure.  But he wanted to do more than get the Bible into people’s hands. He wanted people to read the Bible and understand it—particularly what it says about Jesus Christ.
Klopsch conceived the idea of printing some of the Biblical text with red ink. When reading Jesus' words, "This cup is the new covenant in my blood, which is poured out for you.."
(Luke 22:20), he thought of printing all of Jesus’ words in red, the color of his blood. 
In Modern Christianity, Klopsch wrote in an explanatory note on his red-letter Bible, "...striving zealously to draw nearer to the great Founder of the Faith. Setting aside mere human doctrines and theories regarding Him, it presses close to the Divine Presence, to gather from His own lips the definition of His mission to the world and His own revelation of the Father… The Red Letter Bible has been prepared and issued in the full conviction that it will meet the needs of the student, the worker, and the searchers after truth everywhere." 
In the first red-letter Bible, the words "universally accepted as the utterances of our Lord and Savior" were printed in red.  So were Old Testament passages that Jesus quoted or that were directly related to incidents to which he referred (with the relevant cross reference also printed in red). Old Testament verses containing prophetic references to Christ were identified with red stars, but the entire quote is not in red.
Though an ardent student of Scripture, Klopsch did not attempt to produce the red-letter Bible single-handedly. According to his biographer, Charles M. Pepper, he enlisted many scholars to assist him: "He engaged the services of a number of distinguished Bible scholars, including several leading college professors in this country and abroad. To each of these he wrote explaining the nature of the work, and giving to each a certain part of the books of the Old and New Testaments to mark for the 'Red Letter' edition. When the entire Bible was finished, the separate books were then interchanged among these workers, so that each eminent scholar practically went over the entire Bible and annotated the work already done by others. Many months were occupied in this interchange, but at last the task was accomplished." 
While Klopsch did not, in the red-letter Bible itself, supply the names of the scholars who had assisted him, he did state eloquently the case for such an edition: "Here the actual words, quotations, references and allusions of Christ, not separated from their context, nor in a fragmentary or disconnected form, but in their own proper place, as an integral part of the Sacred Record, stand out vividly conspicuous in the distinction of color. The plan also possesses the advantage of showing how frequently and how extensively, on the Authority of Christ himself, the authenticity of the Old Testament is confirmed, thus greatly facilitating comparison and verification, and enabling the student to trace the connection between the Old and the New, link by link, passage by passage." 
"In the Red Letters Bible, more clearly than in any other edition of the Holy Scriptures," continues Klopsch, "it becomes plain that from beginning to end, the central figure upon which all lines of law, history, poetry and prophecy converge is Jesus Christ, the Saviour of the world. He expounded in all the Scriptures the things concerning Himself and the Divine plan for man's redemption, and the Red Letter Bible indicates and emphasizes this Divine exposition and personal revelation at each successive stage, making them so clear that even the simplest may understand. It sheds a new radiance upon the sacred pages, by which the reader is enabled to trace unerringly the scarlet thread of prophecy from Genesis to Malachi. Like the Star which led the Magi to Bethlehem, this light, shining through the entire Word, leads straight to the person of the Divine Messiah, as the fulfillment of the promise of all the ages." 
Klopsch's red-letter New Testament bore this title: The New Testament... With All the Words Recorded Therein, as Having Been Spoken by Our Lord, Printed in Color. According to Pepper this innovative New Testament met with "instant success."  Klopsch himself proceeded to produce, as we have seen, a red-letter Bible. It was released in 1901 under the title The Holy Bible: Red Letter Edition. Both the New Testament and the Bible were printed by the Christian Herald's own presses and bore the magazine publisher's imprint:
The novelty and success of the red-letter editions of Scripture did not go unnoticed in the competitive world of Bible publishing. At least three major publishers of Bibles quickly added such editions to their lists: A.J. Holman (now Holman Bibles) printed a red letter edition, Thomas Nelson and Sons printed one as well as John C. Winston. Holman, a Philadelphia house, had released The Holman New Self-Pronouncing Sunday-School Teacher's Bible around 1898. Soon after Klopsch published his red-letter New Testament, Holman issued a separate edition of the New Testament portion of its New Self-Pronouncing Bible in which the sayings of Christ were printed in red.  Nelson, located in New York at the time, was the first to provide an entire Bible to compete with Klopsch's. Printed by Berwick and Smith Company in Norwood, Massachusetts, it was titled The Red-Letter Edition of the Holy Bible... showing the Whole of Our Lord’s Words in Red.  Winston, another Philadelphia company, followed suit with The Red Letter Holy Bible, in which "the words spoken by Jesus" and "the prophetic types and prophecies in the Old Testament referring to Christ" are printed in red.  All three of these editions were published during the first decade of the twentieth century.
Red-letter Bibles went on to establish themselves in many quarters as the preferred form of the printed Bible. Louis Klopsch, who died in 1910, would undoubtedly take pride in the endurance of the innovation he introduced into Bible publishing.
1.Hugh A. Moran, "Louis Klopsch," in Dictionary of American Biography, ed. Dumas Malone (New York: Scribner's, 1933), 10:447; Charles M. Pepper, Life-Work of Louis Klopsch (New York: Christian Herald, 1910), pp. 314-316.
2.Pepper, Louis Klopsch, p. 325.
3.Robert W. Funk, Roy W. Hoover, et al., The Five Gospels (New York: Macmillan, 1993), p. 37; Philip Sellew, "Red Letter Bible," in The Oxford Companion to the Bible, ed. Bruce M. Metzger and Michael D. Coogan (New York: Oxford University, 1993), p. 619. Sellew's source was Funk and Hoover, and their source was Laurence S. Heely, Jr., publisher of Christian Herald. Philip Sellew to Allan Fisher, 20 June 1994.
4.Louis Klopsch, "Explanatory Note," in The Holy Bible: Red Letter Edition (New York: Christian Herald, 1901), p. xvi.
6.Pepper, Louis Klopsch, pp. 324-25.
7.Klopsch, "Explanatory Note," p. xvi.
9.Pepper, Louis Klopsch, p. 325.
10.Maragret T. Hills, ed., The English Bible in America (New York: American Bible Society, 1962), p. 326.
11.Ibid., p. 329.
12.Ibid., p. 342.
Funk, Robert W., Roy W. Hoover, et al. The Five Gospels: The Search for the Authentic Words of Jesus. New York: Macmillan, 1993
Hills, Margaret T., ed. The English Bible in America: A Bibliography of Editions of the Bible and the New Testament Published in America, 1777-1957. New York: American Bible Society and New York Public Library, 1962.
The Holy Bible: Red Letter Edition: Containing the Old and New Testaments, Translated out of the Original Tongues… New York: Christian Herald, 1901.
Moran, Hugh A. "Klopsch, Louis." In Dictionary of American Biography. Edited by Dumas Malone. New York: Scribner's, 1933. 10:447-48
Pepper, Charles M. Life-Work of Louis Klopsch: Romance of a Modern Knight of Mercy. New York: Christian Herald, 1910.
Sellew, Philip. "Red Letter Bible." The Oxford Companion to the Bible. Edited by Bruce M. Metzger and Michaeld D. Coogan. New York: Oxford University, 1993. Page 619.
—By Allan Fisher (1994). Used with permission.