Sorry for the gross question.
Why is it sometimes clear and runny and other times it’s green and thick? I thought you have one set of mucous membranes that produce the same mucous all the time. Usually during a cold it becomes think and nasty. Is it the body that is making it thick or is it the cold?
Green and thick means it’s full of bacterial waste products, i.e. you have an infection, up in your nose and/or sinuses.
Mucous is an adjective. The noun is spelled mucus. As in “Mucus is found on the mucous membranes.” One more little nugget of information I could do without.