The priest's robe that he may wear seven days a week, and under his other robes is always black, it is called a cassock or soutane. The robe that he puts on over this for Mass is white, it is called an alb. If he is a religious priest the everyday wear robe is the color is his order, Benedictines wear a black robe, Cistercians wear a white robe, Carmelites and Franciscans wear a brown robe, etc. Any priest is going to be wearing a white alb for Mass.