why is our snot is mostly green or yellowish ? i have read about it in many books but i didn't get any reasonable answer , even a book is named on it " why is snot green ? " .

 

When you have a cold, your immune system sends white blood cells called neutrophils rushing to the area. These cells contain a greenish-colored enzyme, and in large numbers they can turn the mucus the same color.

 

http://www.webmd.com/allergies/features/the-truth-about-mucus?page=2