When you're sick you'll often produce more phlegm, and might notice it's thicker or a different color: white, green, yellow ...

When the nasal tissue gets inflamed, the mucus flow slows down and it gets thick. This can be the beginning of an infection, such as a cold, as your immune system starts mobilising against invaders.

The mucus in your nose serves many functions. Its color can tell you and your doctor a lot about what's going on in your body — especially when it's been an abnormal shade for a long time.