Finn mentions that he was in sanitation, and his knowledge of First Order bases and ships seems predicated on the fact that he cleans up around those bases/ships (which seems like it should be beneath a stormtrooper, but, that's not the point of this question). Does he wear his stormtrooper armor while working sanitation? If not, what does a stormtrooper wear while performing sanitation duties?
As I pointed out in another answer, stormtroopers don't pull sanitation duty, that's something that falls to stormtrooper cadets.
Assuming the training regime is similar to that of the former Empire, a cadet would wear a soft jacket and trousers.