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.

Ambrose was born in Trier (now in Germany) between 337 and 340, to a Roman family: his father was praetorian prefect of Gaul. Ambrose was educated at Rome and embarked on the standard cursus honorum of Roman advocates and administrators, at Sirmium, the capital of Illyria. In about 372 he was made prefect of Liguria and Emilia, whose capital was Milan.

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.