The idea that Mary was conceived without original sin is an old one, even though there is very little basis for it in scripture. It’s the kind of thing that everyone wants to believe, so it must be true.
Prayer: O Mary conceived without sin, pray for us who have recourse to thee. Also, the rosary.
Hymn: Immaculate Mary (which I usually can’t stand).
Colours: white, blue, gold
Flowers: lilies, roses, any flowers