The fun starts when the mucus isn't clear. Almost all doctors agree that cloudy or white nasal mucus indicates a head cold, caused by those nasty viruses that linger in schools, daycare centers and Sesame Street concerts. Head colds may last 5 to 7 days, and typically cause some mild chest congestion before disappearing completely.
Viruses can cause the mucus to be green or yellow.
It depends on the doctor, and that depends on the last thing they've read or how sick the child is. One thing to keep in mind, by the way, is that nasal mucus typically looks dark when dried or in the first 2 hours of the morning, so that doesn't count. And mucus that changes from clear to discolored and back is most likely a virus. Some doctors will take a sample of the mucus and look at it under a microscope for the white blood cells that signify infection. Some doctors have taken to doing limited CAT scans of the sinuses instead, as they are much better as finding sinusitis as well as determining sinus anatomy than conventional sinus Xrays, and usually about the same price.