The truth of the matter is that not all bogeys are green.  They can range all the way from clear down to filthy grey/black.  So why the variety?

First we must identify the standard types of snot, and their common causes.

1.  Clear and runny.  This is usually experienced at the beginning and end of a cold/flu, or can sometimes be triggered by simple environmental factors like going from a cold place to a hot one.
2.  White and sticky.  Tends to stay in the nose and not cause much problem.  Can be a stage of flu.
3.  Green (the stereotypical bogey).  Common during the bunged-up stage of a cold, but also among those who do a lot of outdoor work such as farmers or arborists.
4.  Black and dirty.  People who live in or visit built-up cities will sometimes notice that the pollution builds up.  I like to call this ‘London Nose’.

Standard mucus, as it is produced from the nasal mucosa tissue lining the nose and passages, is colourless.  Since this is the case, what produces the colour?  The list above demonstrates the different stages in the life cycle of the microscopic moss species ‘Bryophyta Microsinus‘.  When there is an irritation in the nose (perhaps a trapped hair, or the early stage of a cold) the mucus runs fresh and clear with no chance for anything to settle in it.  As the mucus solidifies into place, the tiny spores are inhaled and stick in the lining of your nostrils.  The environment is soft and full of nutritious amino proteins, just like a freshly turned flower bed.  The white stage is the root growth of these microscopic mosses.  As they grow and absorb more light, the chlorophyll production increases and they turn a green colour.

They will usually remain content in the nostrils until the bogeys are removed.   This can be triggered as the cycle begins again with irritation, and the passages are rinsed clear by fresh mucus production.  There is also the obvious nose blowing and/or picking.  The third way to end a life cycle is by entering a highly polluted area which will kill the bryophyta and leave them as dead grey matter among the dirt particles.