Is there a historical reason why the liturgical colour purple eventually replaced black to symbolize penance?
Having dived into this question for some time now, I would like to simply postulate a possible response to this solution. At lest until another can come up with a more precise answer.
No historical reason seems to be able to be found at the present moment.
Is it not possible that the cause of this color change was due to how historical black dyes faded into other colors such as purple?
First a little background on the colors involved.
It could be noted that priests who celebrate Mass in the Ambrosian Rite (Rite of Milan) wear black vestments during Lent. Pope Paul VI was the Archbishop of Milan before being elected as Supreme Pontiff of the Catholic Church in 1963.
The liturgical color for Lenten feriae is that of strict penance: black.
In fact, according to the Ambrosian tradition, black is not only the colour of mourning (and, as such, used for requiem Masses), but also the true color of penance and fast: in this sense it is used at least since the XII century not only for Lenten ferial days but also for the Minor Litanies, that in the Ambrosian Rite occur after the Ascension. - Liturgical Colors for Lent in the Ambrosian Tradition
Here are the colors others than black that are of interest in either the liturgy or colors worn by Catholic prelates.
In the twelfth century, Pope Innocent III was the first to specify the colors of the vestments that were to be used for the Roman Rite; almost certainly this reflected prevailing custom in Rome, not an invention on his part. Although a separate subject from this article, it is well to remember that it was only towards the end of the 1st Millenium that the question of vestment color became a significant one. Black was designated for penitential and funeral liturgies, but violaceus was indicated as a substitute for black. Pope Innocent’s treatise De sacro altaris mysterio (Book I, chapter 65, which was written before his election as pope in 1198) seems to be the first indication that violaceus had come to be regarded as a penitential color for the Roman Rite.
Whilst I suggest that it is an error to interpret Innocent’s violaceus as intending only the color violet as we recognize it today, it should also be noted that his treatise in a separate section (Chapter 32) discusses the use of the Mosaic colors (cf Exodus 28:5) scarlet, flax-gold, blue and purple, the latter which Innocent describes as signifying the authority and royal dignity of a bishop. In short, purpura still had the connotation of prestige in the time of Innocent III (quite distinct from a penitential use).
It is likely that at this time (12th century) Tyrian or Imperial purple was still being used in Rome, but it had become the colour used exclusively by the Pope and by nobility. So, when Innocent used the word violaceus, instead of purpura, it would seem very unlikely that he was recommending that violaceus-colored vestments were to be dyed from the expensive process for producing Tyrian purple. That expensive process would be unaffordable and unavailable to Western clergy. Rather, it would seem likely that Innocent’s violaceus was intended as Royal purple , a colour produced from the less expensive non-Murex dyes. It should be carefully noted that these less expensive dyeing processes could produce a violet-coloured dye or an amethyst (or fuchsia) purple coloured dye. But they were colours not so dark as Tyrian purple. I would suggest, therefore, that Innocent’s use of the word violaceus has nothing to do with an attempt to make a distinction (as some scholars have suggested) between the colors we recognize as violet and purple. - The Liturgical Color violaceus in the Roman Rite: Part One and Part Two
These lengthy articles show well that both the colors violet and purple were definitely colors of status. The purple that was reserved for bishops and Monsignori is actually fuchsia or amethyst. Pliny described Tyrian Purple as being the color of clotted blood and thus closer to the scarlet color of cardinals.
We can go on and on, but we seemingly can not find out how purple became associated with penance, remorse or mourning and eventually replaced black as a liturgical color.
Thus in conclusion, I would like to put forth that it may have been due to the fact that black dyes were notorious for fading.
Although somewhat mysterious as to why violet replaced black as a penitential color for Lent. One possible reason for this is that it may have come into liturgical usage due to how black fabrics faded:
Fugitive dyes are unstable. Made from pigments that are not light or color fast, they can fade even if they are well taken care of. One of the most famous examples is this black mourning dress worn by Queen Victoria on the day of her accession to the throne:
Queen Victoria’s Privy Council Dress, circa 1837
That’s a black dress?!
Well, not anymore, but it was.
Originally, this dress was a deep, shimmering black, but the fugitive dye has aged poorly. Black dyes have been historically notorious for fading, usually to this rusty brown. Some black dyes also fade to blue or even purple, depending on the dye used. - Dying Dyes: What You See Isn’t Always What Was
Could it not simply be that parish churches in past centuries could not afford new vestments and continued to use their black vestments, even after they lost their black coloring and faded purple or some shade of purple thereof?