The reason it turns green is because there now are white blood cells in the discharge fighting the infection. If the child is not improving in 7-10 days with the cold and cough, you need to be concerned that there may be a more serious infection. Sinusitis, that is an infection of the sinuses, may occur if the runny nose and cough continue to not improve after 10-12 days. If such is the case, you should contact a physician. My child has been diagnosed with pneumonia.
Pneumonia is an infection in the lung tissue.
As with all childhood infections, viruses are the most common cause especially in the very young child. Because so many pneumonias in the young child are viral, these will not respond to antibiotics and simply have to run their course. Older children are more likely to have an infection caused by a bacteria or an organism called mycoplasma and will need antibiotics. Appropriate treatment is a decision that will be made by your physician. I've been diagnosed with Bronchitis and treated with an antibiotic.