About the general rule of color(s) in clothing in the years between 700 and 1600 AD… at times there were restirctions on who could wear what colors, by social class. Its a very general reason that monks who leaving the ‘attachements’ of the world took brown or black for their most common color or prehaps the natural color of the fibre… White, red, purple and black were for centuries restricted to the nobles and upper gentry…At a time when ‘favor’ with the Church (or rather the local clergy of liberal and /or morality and greed instinct) could be bought with
‘gifts to the church, or at times the designation of a son or daughter, to the dedication of sacred life. So, the clothing rules often followed them into their new and humble life of service( noting that in later years the more vows a person took the more restrictive the attire and dress code)… remember I am condensing over 800 years. In some bastions of the faith it was desiginated by rank in the ecclesiastical format that certain yet still noble colors would/could be assigned and given signifigance. The whims and reasonings of various Papal persona also playing a part. The Pope in order to be identified would commonly wear bleached and pureset whites possible, in the best available textiles. Others would wear red or a purpley aramanth (red family but distict from cardinal or royal red.) the pastors of the congregants who were most in contact with the people would wear black, it was lovely and modest and suitable and identifiable to the task at hand… all this helped the literate and the illiterate immedialty identify various levels of the clergy…and as you stated above it’s not a hard fast rule… but rests more to the particul vows and society of, and geographic location of, the wearer.

I think grays have always been the most utilitarian dyes because one needn’t use too much to achieve a shade of gray, whereas black would require a lot of dye and white a lot of bleach.

Just my 3 cents.

I wish I could give you a reference for this, but I’ve encountered it over and over again in my reading regarding medieval monasticism (overwhelmingly the largest number of clerics for centuries). I suspect that what was easiest, cheapest and practical was also seen as imbued with theological value (humility, etc, and all that). Brighter colors only came along when affordable (which bishops and those with higher-paying benefices generally had).