My son has had green snot for over 2 weeks. No fever, no pain, good temper. Is this just a long-lasting cold or something more serious?

Green nasal secretio. Haveing green nasal secretions for over two weeks may be due to an infection in the nose or sinuses. It is best to see your pediatrician for an examination to determine the cause and to get proper treatment.
Probably Not Serious. Dr pizzo's answer is correct. However the "greenness" is not always indicative of infection. Allergic rhinitis can have green drainage as can a foreign body in the nose. See your doctor.
Allergies. I tell my moms, what goes in comes back out, so esp if in high vol pollen area that may be all it is, .