Other mistake: In the scene where Betty and Joan are in the kitchen after dinner, we see the coffee percolating in the pot but never see Betty pour the coffee into the china pot before taking the tray into the living room.
Other mistake: Betty's mom is requesting a change in the table setting, eg. no candlesticks, glassware trimmed with gold etc. You then see the actual wedding happening and see candlesticks and glassware without the gold rim.
Other mistake: When the girls are sitting in the common room on the night Giselle shows them the contraceptive, Betty says, "Do you realise November 2nd is three weeks from now?" Yet, the University newspaper with Betty's nasty editorial shown on screen the next day is dated September 19th. (00:18:43 - 00:23:53)
Answer: The colour change every year and the number is the year they finish (Joan's hat says 54 and their last year is 1954 as Nancy says around new year.)