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.
Rosh Hashanah and Yom Kippur. Yom Kippur is the tenth day of [the] seventh month (Tishrei) and is regarded as the Sabbath of Sabbaths. Rosh Hashanah (referred to in the Torah as Yom Teruah) is the first day of that month according to the Hebrew calendar. On this day forgiveness of sins is also asked of God.