First, Jesus didn't address a lot of topics based on what is written and attributed to Him in the New Testament. He usually taught based on the audience and their questions. For example, in Matthew 19 the Pharisees were trying to trick Jesus by asking about divorce. Jesus as usual turned it around on them while also giving us the best understanding of the topic of divorce anywhere in Scripture. Specific topic, specific audience but a timeless truth.
A topic that is often cited for being unimportant because Jesus didn't address it is homosexuality. Jesus didn't say that homosexuality is wrong so it must be OK. Not really. Jesus often spoke on a topic if the understanding of it was skewed, like the issue of divorce or the eye for eye system of justice. His silence on a topic, speaking as a Jew to a largely Jewish audience, would rather than declaring a topic unimportant would instead leave the accepted understanding in place. No one was going around asking if homosexual behavior was OK because everyone already knew it was condemned in the strongest possible terms. Jesus didn't teach "You have heard it said that homosexuality is an abomination but I say unto you that it is OK if it is done in a committed relationship". Those being taught by Jesus would have come to the conversation knowing homosexuality was an abomination and nothing He is recorded as teaching would have altered that understanding.
Second, Jesus said a lot of stuff that is recorded in the New Testament but you can read everything written in red in what, an hour or maybe two? But Jesus lived for many years. He certainly wasn't silent except for the words in red in our English New Testament. The apostle John wrote:
Now Jesus did many other signs in the presence of the disciples, which are not written in this book; but these are written so that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name. (John 20:30-31)
Now there are also many other things that Jesus did. Were every one of them to be written, I suppose that the world itself could not contain the books that would be written. (John 21:25)
The teachings of Jesus that were preserved are a fraction of what He taught but they are specifically preserved for our benefit. So when Jesus speaks to the centurion and doesn't tell him to love his enemies and to lay down the sword in what we have recorded, that doesn't mean the topic never came up. Arguing from the alleged silence of Jesus when we expressly know He taught a ton of stuff that is not recorded is intellectually dishonest.
Third, Jesus is not the only person quoted in the New Testament, far from it. Much of what we know of the Gospel, of the Kingdom, of the cross, resurrection and atonement, of justification, of election, of the nature of Jesus Himself, we know from other sources than Christ.
Jesus spoke and taught almost exclusively pre-cross. His teaching therefore focused heavily on the prophetic fulfillment of His incarnation, on the impending inauguration of the Kingdom, on correcting misunderstandings about the Law and on establishing Himself as the great I AM, the Messiah. After the cross His disciples taught extensively on the meaning of the cross, on the Gospel of justification by faith via the substitutionary death of Christ and especially on how the church post-cross should relate to each other and the world and specifics on issues that rose up (Do we need to keep the dietary laws? What about circumcision? Who should lead the church? Why do so many Jews not believe?). most often those topics are addressed by the most prolific NT writer, the apostle Paul This brings us to a second evasion tactic in this category, pitting Jesus against the writers of the non-Gospel account books of the New Testament, most especially Paul.
If we try to jettison Paul we are going to have a pretty small New Testament and worse we will have an incomplete and ineffective New Testament. Those who slander Paul do so because that is easier than dealing with what he wrote. It is perceived to be better to argue from the unrecorded alleged silence of Jesus where we are free to fill in the blanks than it is to wrestle with what an apostle of Christ had to teach us that he based on what Jesus taught him.
The New Testament is not "Jesus or Paul" it is "Jesus and Paul (and Peter and John and Luke etc.)". If you truly want to know Jesus better so you can follow Him more faithfully then you absolutely must read Paul and the other apostles. In fact I would suggest that telling people to discount Paul is tantamount to being a false teacher. Paul knew Jesus better that almost anyone who has ever lived and if we want to know Jesus better there is no better teacher, other than Christ Himself, than Paul. Sitting at the feet of Paul is one of the best ways to learn to love and follow our Savior.
When someone tells you an absence of letters in red on a topic means it is unimportant, remember these three points. The idea of a lack of red letters equating to unimportance is an easy and quick way to sidestep difficult topics, allowing one to come independently to their own conclusions, but it makes absolutely no sense when viewed in light of the New Testament.