Luca - color of mucus:
Generally nasal mucus is clear and thin, serving to filter air during inhalation. During times of infection, mucus can change colour to yellow or green either as a result of trapped bacteria, or due to the body's reaction to viral infection.
In the case of bacterial infection, the bacterium becomes trapped in already clogged sinuses, breeding in the moist, nutrient-rich environment. Antibiotics may be used to treat the secondary infection in these cases, but will generally not help with the original cause.
In the case of a viral infection such as cold or flu, the first stage and also the last stage of the infection causes the production of a clear, thin mucus in the nose or back of the throat. As the body begins to react to the virus (generally one to three days), mucus thickens and may turn yellow or green. In viral infections, antibiotics will not be useful, and are a major avenue for misuse. Treatment is generally symptom-based; often it is sufficient to allow the immune system to fight off the virus over time.
So mucus should be clear (beginning), turn to yellow/green (when body reacts to the virus) and back to clear (recovering).