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   It is not so much the snot which is green as the phlegm component, and this is only when it is infected. Healthy phlegm is not really green.The phlegm becomes green in the presence of infection (bacteria) due to the large numbers of white blood cells present, which make their way through the airway walls to combat the infection. White blood cells also give pus a whitish-green color, so you'd think mucus would be white but it's snot. (Get it? Sorry, but I couldn't resist.)

There's more to it. Many of the common bacteria associated with humans have color. Staphylococcus aureus is a golden yellow, and Pseudomonas pyocyanea is a shade of blue. The green color of snot comes from the mixture of the yellow and blue. Interesting, yet gross, and hopefully inspirational to stay vigilant about not catching a cold.

http://www.fitsugar.com/Why-Boogers-Green-139800

• Clear mucus If your boogers are clear, you're in good shape. Clear is the natural, healthy color of your mucus.


• Yellow mucus If your snot is yellow, you may have a problem. Yellow mucus often signifies something damaged up in nose-land. You may have a sinus infection with a blocked nasal passage, which can turn into chronic sinusitis. You may have nasal polyps or even bronchitis. If you have yellow boogers for several days -- especially with sinus pain -- you should probably call your <font style="color: #da086b ! important; font-family: inherit ! important; font-weight: inherit ! important; font-size: inherit ! important; position: static;" color="#da086b">doctor</font>.


• Green mucus Slimy green boogers are just as bad as yellow ones. Bacteria are usually to blame for green snot, but you may also see the color change to green from irritants like smoke or pollution. You may find green stuff coming out more when you cough than when you blow your nose, but it can come out either way. Out is always better than in! Chances are, if you're coughing up green stuff, you've got an inflammation in the bronchial tubes in your lungs.


• Brown mucus My boogers are a gross shade of grey or brown if I've spent the evening out at a smoky bar. If your sinuses are swollen, you may find blood mingling with your boogers, turning them icky dark colors. Smoking and drinking alcohol can make things worse.


• White mucus White is probably the rarest of all booger colors. But don't think you've won the jackpot if your snot is white, unless you're hoping for a <font style="color: #da086b ! important; font-family: inherit ! important; font-weight: inherit ! important; font-size: inherit ! important; position: static;" color="#da086b">medical</font> jackpot. White mucus can be caused by certain dairy foods. If you are experiencing congestion and pressure in your sinuses and drink milk, you may see your mucus turn white.

This list is not intended to diagnose any problems. If you are concerned about the things and colors coming out of your nose, call your doctor!

http://health.families.com/blog/the-secret-language-of-boogers#