The Latin phrase on Northwestern's seal, Quaecumque sunt vera (Whatsoever things are true) is drawn from the Epistle of Paul to the Philippians 4:8, while the Greek phrase inscribed on the pages of an open book is taken from the Gospel of John 1:14: ο λόγος πλήρης χάριτος και αληθείας (The Word full of grace and truth). Purple became Northwestern's official color in 1892, replacing black and gold after a university committee concluded that too many other universities had used these colors. Today, Northwestern's official color is purple, although white is something of an official color as well, being mentioned in both the university's earliest song, Alma Mater (1907) ("Hail to purple, hail to white") and in many university guidelines.

Answer this question, if possible (if impossible, reply "unanswerable"): What has been Northwestern's official color since 1892?
Purple