You might have heard that yellow or green mucus is a clear sign that you have an infection, but despite that common misperception, the yellow or green hue isn't due to bacteria. 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. But you can have perfectly clear mucus and have a terrible ear and sinus infection , Kao says.If you do have an infection, you'll likely also have other symptoms, such as congestion, fever, and pressure in your face, overlying the sinuses, Johns says.f you've ever stopped to look at the contents of the tissue after you've blown your nose, you may have noticed that your mucus isn't always perfectly clear. It may be yellow, green, or have a reddish or brownish tinge to it.