It’s gross enough to be producing mucus, and it gets progressively worse when it starts turning yellow, red or even black. Yuck.

Thankfully, it’s possible to decode what these colors mean and how you should react. But first, it helps to know a little bit about mucus and how it works. Mucus serves as a protective barrier between the outside world and the internal workings of your body. Mucus membranes pump out this sticky fluid – which is clear in times of good health – to help stop pollutants from entering your respiratory system. The body makes about a liter of mucus and phlegm per day, and there’s a slight difference between the two. Phlegm is made in the lungs and trachea, and mucus is made everywhere else in the respiratory system.

Depending on the kind of pollutant or illness you’re facing at the moment, your body and your mucus will respond differently. Here are a few of the potential mucus colors you might see in your facial tissue and what each means:

Clear – If it’s clear, you’re probably in the clear. Most of the time, mucus ends up in the stomach, where its proteins are digested and reused elsewhere in the body.

White – White mucus is a sign of congestion. It might mean you have a cold.

Yellow – When white blood cells attack viruses and bacteria and then die, mucus collects them and carries them through the body. Dead cells of this type provide a yellow hue.

Green – Much the same as the condition outlined above for yellow, except green mucus indicates your body is working even harder to rid itself of foreign invaders.

Pink or red – Pink or red mucus means it’s carrying blood. The blood is likely from the blood vessels in your nose. A spot of blood might be okay if you have an accompanying cold, but bloody mucus could be a sign of something more serious. Check with your doctor if this is your symptom.

Black – Although it could be the sign of a fungal infection, black mucus is more likely to be discolored by particulate matter in the air or smoke.

Always consult your physician or care provider for medical help.