As a fan of his work, I'll be the first to say Nas hasn't always secured the best production for his albums. He certainly has substantial highlights ("N.Y. State of Mind", "The Message", "Nas Is Like" to name a few), but there have definitely been moments of his career which suffered due to substandard instrumentation.
"Purple" is not one of those moments.
For me, the most unique element of this track is it's lack of a hook (or chorus to some). Hooks are the staple of hip hop songs, usually because they are the only words anyone can remember after a first listen. They are also the lyrics to which people outside of the culture tend to gravitate. I'll never forget a stroll I took on the campus of Louisiana State University through the tailgating festivities before a football games. I walked past one group of fans who were flying a purple and gold Confederate flag and listening to OutKast's Stankonia album. I watched with slacked jaw as they flailed and gyrated while singing along, chanting "I'm sorry Ms. Jackson!" Surreal.
I think it's actually quite daring for Nas to forego adding a hook on this track. The urge to "catchify" a hip hop song is significant, and many otherwise decent songs have been weakened with the addition of weak or unnecessary "hookage". With "Purple" Nas recognized the obvious density of what he's written and decided it was enough to carry the song. That type of commitment to one's message is difficult to play; ask any actor who has moments alone on stage. But because Nas has chosen to simplify this composition, his words become much clearer and more expressive.
I dig it.