Do you have a cold? 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.
You can also thin out the mucus with guaifenesin, a type of medicine called an expectorant. Thinner mucus is easier to get out of the body. Possible side effects of guaifenesin are dizziness, headache, nausea, and vomiting.