A Purple Moon occurs when a New Moon falls at least some 21 days before the Northern Winter Solstice. It is similar to Violet Moon, in such that it is moon that falls on the cusps between the New Years.
Nobody can give a perfect distinction between Violet and Purple but they are similar in this case they both relate to the Moon of Last Year that has hung on to the new year.
Basically when the New Year comes, we are celebrating a lap around the sun. But you gotta' remember that the moon of the new New Year was most likely created last year so, at least at night, when the sun is hiding, the Old Moon from 'Last Year' is still shining on.