http://health.families.com/blog/the-secret-language-of-boogersJames_Sunderland said:I once sneezed purple boogers. I never found out why.
while it doesn't say anything about purple... it does say see a doctor and just ask.
http://www.merck.com/mmhe/sec19/ch221/ch221g.html
Vasomotor Rhinitis: Vasomotor rhinitis is a form of chronic rhinitis. Nasal stuffiness, sneezing, and a runny nose?common allergic symptoms?occur when allergies do not seem to be present. In some people, the nose reacts strongly to irritants (such as dust and pollen), perfumes, and pollution. The disorder comes and goes but is worsened by dry air. The swollen mucous membrane varies from bright red to purple. Sometimes, people also have slight inflammation of the sinuses.
rhinitis = a nasal allergic reaction... so I'm guessing you a weird allergy to something.