Jesus' conversation with Nicodemus ends with John 3:12.

They may have gone a little overboard with the "words of Christ in red" thing.

The people who make those red-letter Bibles generally assume that John 3:10-21 is a single long quotation of what Jesus said to Nicodemus. The New Revised Standard Version places its quotation marks in accordance with that interpretation, too, but mentions in a footnote that "some interpreters hold that the quotation concludes with verse 15." I hold that even those commentators haven't been stingy enough with the red ink, and that the quotation concludes with v. 12. Here are my reasons for thinking so.

Verse 13 reads "And no man hath ascended up to heaven, but he that came down from heaven, even the Son of man which is in heaven." This clearly states that the Son of Man (Jesus) has already ascended to heaven and is in heaven now. This was true at the time that the Gospel was written, but it was very obviously not true at the time of the conversation with Nicodemus -- when Jesus was, well, having a conversation with Nicodemus. You know, on earth.

"And no man hath ascended up to heaven" (v. 13) -- a blanket statement that ignores such obvious counterexamples as Enoch and Elijah -- reminds us of the similarly categorical statement in John 1:18, "No man hath seen God at any time," which ignores the obvious counterexample of Moses. John 1:18 is clearly the author writing in his own voice, not quoting Christ, so it makes sense that the same would be true of John 3:13.

"For God so loved the world, that he gave his only begotten Son" (v. 16) sounds as if God had made a great sacrifice by "giving" his Son. This makes the most sense in a post-crucifixion context, when readers would understand that God so loved the world that he had allowed his beloved Son to be tortured to death.

If vv. 10-21 is a single long speech to Nicodemus, it is strange that nothing is said of Nicodemus's reaction, or how the conversation ended. The narrative just abruptly stops, and we jump to the next episode in Jesus' life. It makes much more sense if vv. 9-12 are the end of the conversation.

[9] Nicodemus answered and said unto him, How can these things be? 

[10] Jesus answered and said unto him, Art thou a master of Israel, and knowest not these things? [11] Verily, verily, I say unto thee, We speak that we do know, and testify that we have seen; and ye receive not our witness. [12] If I have told you earthly things, and ye believe not, how shall ye believe, if I tell you of heavenly things?

This certainly sounds like a refusal to teach this "unbeliever" any more, not like a prelude to further holding-forth about Moses lifting up the serpent and God so loving the world.

The content of vv. 13-21 is not clearly related to the preceding conversation about being born again; the most reasonable assumption is that it is a bit of commentary by the author (the Beloved Disciple) speaking in his own voice.

The only potential problem that I can think of for this interpretation is v. 14: "And as Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up." This "lifting up" of the Son of man is most naturally interpreted as a reference to the crucifixion -- but the author can't very well be saying "Jesus must be crucified" after the crucifixion and resurrection have already taken place. The choice of words does probably allude to the crucifixion, but the primary meaning must be something else. He must be saying something like: We must "lift up" the Son of man -- i.e., spread the word about him, preach the gospel -- so that others can believe in him and have eternal life.