Dom Gueranger talks about Christmastide as ending with today's festival of the Purification/Presentation. Certainly, the rules concerning the Antiphon of our Lady after the Divine Office would suggest that.
The C of E, which so often goes over the top in its ultrapapalism, goes even further than Gueranger. The liturgical colour until February 2 is to be white. And they have invented a contorted oddity called the Epiphany Season, modelled on the Easter Season of the Novus Ordo. So the period from Epiphany to Presentation is not allowed to have "Sundays after Epiphany"; they have to be called "Sundays of Epiphany". (They have needed to keep fidling with their Lectionaries because when Epiphany falls on a Sunday, that is itself the "First Sunday of Epiphant; and so the next Sunday is "The Second Sunday of Epiphany" ... but which propers does it have?) Of course, one continues to use the Epiphany Preface. Their view is that February 2 is when we stop thinking about Nativity themes and set our faces towards Jerusalem; towards Passion, Death, and Resurrection.
I believe some medieval sequences of colour may have stayed in white until February 2, but certainly not all (Exeter: viridibus est utendum).
The most obvious solution is to conclude that the Western Churches have never quite sorted all this out. For wise people who follow the old Roman Rite, the definitive moment is surely when the Gesimas start. That can be as early as 18 January or as late as 21 February. It would be very messy to get to work on the magnificent themes of Septuagesima on 18 January while trying to pretend that we are still in 'the Christmas Season'.
So, in my view, the Vetus Ordo gives us the most logical solution. The second most clean and logical system is, surely, that of the Novus Ordo: go green on the day after the Feast of the Baptism and stay green until Ash Wednesday brings down the shutters on "Ordinary Time" and viridity. The Church of England provides us with an example of the almighty mess what happens when one tries to be too clever, which is the invariable temptation of gathering together 'Liturgical Experts' and calling them a Commission.
(The man responsible for the C of E jiggery pokery was 'Bubbles' Stancliff, quondam Bishop of Salisbury, who was always having Bright Ideas. Another of these was to call the weeks before Advent the Season of the Kingdom, and to wear royally red vestments. General Synod drew the line against this adventure in nomenclature. It would, to be fair, have joined up the old and new dates for Christ the King.)