The color of nasal mucus can be used as an indicator to tell us what is going inside out nasal passages, the normal color of nasal mucus tends to be watery and clear, which means that any change in the color of nasal mucous could be a sign for certain health problem. So, through this topic we will highlight the most significant abnormal colors of nasal mucus as well as their pathological connotations.
1.    White color of nasal mucus: it occurs when the mucus become more thick, which denotes swelling and congestion of the nose, as in common cold and influenza. 
2.    Yellow color of nasal mucus: it indicates an early stage of microbial infection.
3.    Red color of nasal mucus: it means nasal mucus is tinged with blood, which may occur due to rupture of small nasal capillaries or damage of nasal tissue.
4.    Green color of nasal mucus: it indicates late stage of infection, especially in sinusitis or chronic rhinitis.
5.    Black color of nasal mucus: it's the usual color of nasal mucus for heavy smokers, but it also could be a sign for fungal infection.
Anyway, if you noticed any abnormal color of nasal mucus, it's advised to seek medical advice immediately.