Prior to the Second Vatican Council, unbleached candles were a more common sight in parishes, though typically reserved to Requiem Masses and Good Friday. However, some liturgists of the late 19th and early 20th centuries even recommended unbleached candles to be used during Advent and Lent.
For example, in The Irish Ecclesiastical Record in the late 19th century, the question of using unbleached candles was answered according to the rubrics at the time.
Using unbleached candles during Lent was never an obligation, but it was also never forbidden. Current liturgical rubrics do not mention the exact color of the candles, and the USCCB only mentions that they can not be electric lights or oil lamps.
Symbolism of unbleached candles
Alban Butler, in his book The Moveable Feasts, Fasts, and Other Annual Observances of the Catholic Church, briefly mentions the symbolism of these unbleached candles.
Generally speaking, candles represent the light of Jesus Christ, and during Lent the Church is in mourning until the great feast of Easter, when the bright white of the Easter candle is brought into the darkness of the church.
While this tradition is not widely adapted by many parishes in the world, it does provide another way to highlight the glory of Easter and to prepare our hearts for the coming feast.