In the first few days of a cold, the mucus coming out of your child’s nose will usually be clear. By the 3rd or 4th day, it’s common for this mucus to turn yellow or green. Parents may misinterpret this to mean that the child has a bacterial infection. The reason your child’s nasal mucus turns color is because her body’s defenses begin to kill the virus. As white blood cells “ingest” viral particles, they die and mix in with the clear mucus creating a lovely shade of yellow or green. This process will go on for three or more days until the viral infection is eliminated. Doctors become concerned about sinus infections if a child is very sick or if the infection goes on too long (usually more than 10 days) regardless of the color of the nasal mucus.