White (gold) - celebration, festive feasts, purity, e.g Christmas and Easter, feasts of Christ
Red - the blood of martyrs, feasts of the Holy Spirit, occasions of the Holy Spirit (confirmation, ordinations, etc)
Violet - penitential seasons or days e.g. Lent, Advent - may be ameliorated or lessened in nature by old rose (aberrantly referred to as pink ;-)) on 3rd Sunday in Advent, 4th Sunday in Lent [sometimes referred to as Rose Sundays as often used to be the days on which the Pope created new cardinals] - often ameliorated in penitential nature during Advent by a blue (often called Sarum Blue) for days of Our Lady, seasons of Our Lady, etc. (in Lady Chapels often the sanctuary lamp will blue rather than red)
Green - seasons of new and/or renewing life - season of Pentecost or “Common Time”
Black - rarely used any more, thank God! - funerals or Good Friday - now most of Christendom rightly recognizes funerals as celebrations of resurrection and therefore Paschal in nature rather than somber, mournful occasions - and uses white or gold festive colors and vestments for funerals, etc.