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 or the Day of Atonement is the most solemn and important holy day of the Jewish calendar. In the Old Testament, the Day of Atonement was the day the High Priest made an atoning sacrifice for the sins of the people. This act of atonement brought reconciliation between the people and God.