Liturgical colours are the practice of matching vestments, altar hangings and other church furnishings to the specific season or feast of the church calendar, a visual system that lets a congregant read the church year at a glance from the colour standing at the altar before a single word of the service has been spoken.
Colour usage was not standardised for centuries of Christian practice, and the Roman scheme most widely recognised today, violet for Advent and Lent, white or gold for Christmas and Easter, red for Pentecost, martyrs' feasts and Palm Sunday, and green for the ordinary weeks outside these major seasons, was substantially codified in the writing of Pope Innocent III around the turn of the thirteenth century, whose treatise on the subject became a major reference point for subsequent Western liturgical practice even as full uniformity across dioceses and religious orders took considerably longer to achieve and, in some respects, was never total. Individual colours carry theological association beyond simple seasonal marking: violet signals penitence and preparation, white and gold signal joy and triumph, red signals the Holy Spirit's fire at Pentecost and the blood of martyrs, and green, the colour of growth, marks the long stretches of ordinary time given to steady spiritual growth between the calendar's more dramatic seasons.