Q: Can a priest where white vestments as an alternative color on any day of the year?
A: Only if he is a concelebrant. Redemptionis Sacramentum 124 says that the principal concelebrant should always wear the chasuble of the prescribed color. Other concelebrants may wear white chasubles if there are insufficient chasubles of the proper color. If even those are not available, the other concelebrants may wear only the stole over the alb.