They were martyred at Carthage in 203 during the persecution of Septimius Severus. With so many martyrs of the third and fourth centuries we have to say “they were martyred but nothing else is known about them.” That is not the case here. We have a detailed contemporary account of their arrest, trial, sufferings and martyrdom, written partly by the saints themselves and partly by an eye-witness. Devotion to them spread rapidly and they are mentioned in the Roman Canon of the Mass. See the articles in the Catholic Encyclopaedia and Wikipedia. The Wikipedia article contains links to the original account of their martyrdom.

Tertullian was a prolific early Christian author from Carthage in the Roman province of Africa. Like St Augustine of Hippo some two centuries later, he was of Berber origin. His is the first substantial body of Christian writing in Latin, the language of the western Roman Empire. He played an important part in the shaping of Christian doctrine, which is to say, taking what happened in the life, death and resurrection of Jesus, and working on understanding it, making sense of it, even inventing the words with which to make sense of it – such as “Trinity”. He was a noted apologist, or explainer of the faith to non-Christians, and in a predominantly pagan world he was insistent against diluting Christianity by compromise with paganism. He eventually found the Catholic Church too lax in its ways and too ready (for example) to forgive those who had apostasized in time of persecution, and he joined the puritan Montanist sect. Nevertheless, his work was an important step in the hammering out of doctrine and St Cyprian referred to him simply as “the Master”.

Violet is a dark colour, ‘the gloomy cast of the mortified, denoting affliction and melancholy’. Liturgically, it is the colour of Advent and Lent, the seasons of penance and preparation.