My point is not to present a long history of the colors here or to insist upon one practice or another. The colors of the season were not written in Scripture or given to the Church with direct revelation to one of the Apostles. The use of color evolved with the major choices laid down between the sixth and twelfth centuries -- with variants in different places. (Though some wags insist the whole thing is a marketing conspiracy by Almy to get parishes to part with money!).
The color of Advent is a church usage about which differences should not cause problems. It is precisely this latitude that adiaphora has in mind. Though some of you might not believe it, I am not very fussy on this point. It is a point about which people may disagree and this disagreement cause no problems at all. My first parish used purple. My present parish uses blue (although a deep indigo blue that distinguishes Advent from the lighter blue usually associated with festivals of the Virgin Mary).
I had a person wonder why we had blue and I have had people wonder why we do not have purple but I have never had a person ask why not the pink of Advent III. So colors are not set in concrete but are part of the evolving tradition of the church. There is nothing wrong with this but this is something wrong about insisting that one path or another be the only choice. Actually, in this day and age, I am happy just to see paraments in use at all (or vestments worn by the clergy!).