The nasal mucosa continuously produces minimal amounts of discharge in order to keep the surface of the upper respiratory airways moist. This nasal discharge is necessary for lung health and the proper function of your respiratory tract. It is, however, clear and liquid — and under normal circumstances, you will probably not notice it at all. 
You probably associate the word "snot" with changes in your nasal discharge that indicate a health problem. Alongside a change in the color of your snot, you'll experience nasal obstruction — a "stuffy nose". A stuffy nose is one of the most common health complaints out there, with infections, allergies, structural abnormalities, and vasomotor rhinitis being the prime causes of nasal obstruction. Most human beings experience the common cold, one of the top culprits of a stuffy nose, up to three times annually. 
A stuffy nose alone doesn't mean you need to see your doctor. When you experience a fever, considerable discomfort, your symptoms last for longer than 10 days, or your snot's characteristics are such that they indicate an infection, it is, however, time to seek medical attention. The color of your snot alone isn't enough to make a reliable (self-) diagnosis, but it can be a useful tool for patients who are deciding whether or not to make an appointment with their family doctors.
What White Snot Says About Your Health
If your snot turns white, it might point to the presence of some kind of inflammation in your nasal mucosa. The cause of the inflammation is usually an infection, which can be bacterial, viral, or fungal. The inflammation causes a thickening of the snot and makes it look cloudy. An intensely white snot color is suggestive of a fungal infection, most probably by Candida albicans or Aspergillus species. 
What Does Yellow Snot Mean?
Yellow snot usually points to a bacterial infection, with the most common bacteria — different types of Streptococcus and Staphylococcus  — affecting the upper-respiratory tract. Bacterial infection causes migration of leukocytes, or white blood cells, towards the place of the infection. After they pick up and destroy the bacterial cells, some of them get exhausted and “die”, which gives your snot that lovely yellow color.
Why Do You Have Green Snot?
Green snot almost always points to a bacterial infection. Green snot can appear as the result of any bacterial infection, but whenever your snot turns green, a Pseudomonas  should be suspected. In this case, a nose and throat culture with an antibiogram is necessary in order to identify the bacteria and to determine which antibiotics will best fight your nasal infection.
I Have Bloody Snot: What Now?
The appearance of blood in the snot is most commonly the result of irritation. Your nose may become irritated because you're dealing with an infection or an allergy, and because you wipe, pick, or blow your nose frequently. The small blood vessels of the nasal mucosa are incredibly sensitive, and easily become disrupted — with blood-stained snot being the result. If small amounts of blood are expelled from the nose, with no symptoms of allergy or infection, the cause can be increased blood pressure, mechanical injury of the fragile blood vessels of nasal mucosa, or even nasal polyps. 
What Else Do You Need To Know About The Appearance Of Snot?
You may have grown up with your parents or grandparents telling you that strangely colored snot was a sign you needed to see a doctor — most likely for a course of antibiotics — but color isn't the only important and informative property of snot.
- Even clear snot can indicate a pathological process, such as allergies or the beginning of a viral infection if it's abundant.
- Snot consistency also matters. Very thick snot suggests an intense bacterial infection or perhaps dehydration.
- Snot has no odor in normal circumstances. Your snot only takes on a bad odor when an infection is present, and the odor varies depending on the type of the bacteria causing the infection.
What About Phlegm (Or Sputum)?
Almost all of these facts are also true for sputum. One of the differences is that coughing up bloody sputum can be a symptom of more serious disorders such as lung cancer , so it requires deeper investigation. However, a small amount of blood in sputum during respiratory infections with intense coughing is fairly common. The overproduction of phlegm can also point to chronic obstructive pulmonary disease (COPD) or asthma .
Sticky, unclear, and thick snot and sputum, which appear during childhood and do not resolve, may be a symptom of Cystic fibrosis , which is a genetic disorder resulting in disrupted chlorine secretion by exocrine glands in the whole body.