I know this is a couple months old, but I am having the same problem...I started coughing up stuff back in May after aspirating sea water while scuba diving. At first, it was bright yellow...the ship's doctor didn't think anything of it.
I saw my regular doctor when I returned home and he wasn't concerned either. My lungs sounded clear, so he told me it was allergies (even after I told him I aspirated sea water) and told me to make sure to take my allergy medicine every day.
I've been doing that since that time for months now and nothing has changed. I keep coughing crap up out of the upper area of my lungs and it never stops. I am sick of feeling this way...I cough constantly. Sometimes, I have to make myself cough very deeply and forcefully to get the gunk out of my throat as it is very, very thick.
I can't go to a different doctor as I receive healthcare through the VA right now and do not have insurance, so that is not an option.