In general, nasal mucus is clear and thin, serving to filter air during inhalation. During times of infection, mucus can change color to yellow or green either as a result of trapped bacteria or due to the body's reaction to viral infection.
As such, there is to be no work done on this day. There are other restrictions, as well. Unless health dictates otherwise, Yom Kippur is observed by a 25-hour fast that begins prior to sunset on the evening before Yom Kippur and ends shortly after nightfall on the day of Yom Kippur. During this time, you are not allowed to consume any food or beverage, including water.
Yom Kippur, then, is one day in length but spans two calendar days, the 9th and 10th days of the Seventh Month. If the day began at evening, then Moses would have simply been told that Yom Kippur or the Day of Atonement was on the 10th day. But he wasn't. He was told to start it the evening before on the 9th day. In other words, the evening part of Yom Kippur belonged to one day (the 9th) and the morning part belonged to another day (10th), breaking the usual tavnith (pattern) for the Sabbath Day and for all the other ordinary and festival days of the year.