Catholics mark ash wednesday in black

That's correct! Catholics mark Ash Wednesday, which is the first day of the Christian season of Lent, by wearing ashes on their foreheads in the shape of a cross. This is a traditional and symbolic act of penance and mourning, and is meant to remind the faithful of their mortality and the need for repentance.

The ashes are typically made from the palm branches that were blessed on the previous year's Palm Sunday, and are imposed on the forehead by a priest or other authorized minister during a special Ash Wednesday Mass or service. The ashes are usually marked in the shape of a cross, and are often accompanied by the words "Remember that you are dust, and to dust you shall return" (Genesis 3:19) or "Turn away from sin and be faithful to the Gospel" (Mark 1:15).

Wearing ashes on Ash Wednesday is a way for Catholics to publicly demonstrate their commitment to prayer, fasting, and almsgiving during the Lenten season, and to prepare themselves for the celebration of Easter.