I don't know about the colouration of your discharge, however I would suggest that the discharge itsself is some form of pus/dead bacteria as per normal, but in greater quantity.
You indicate that your were taking antibiotics both times this has occurred, and whilst they were for adult acne, they will also kill any other bacteria that may be sensitive to their presence.
I would the inclined to think that what you have experienced was a discharge of some form of concurrent nasal infection, whether it was bacteriological or fungal.
From my on-line research in the past: green, grey and/or black discharges are associated with, but not limited to, fungal infections.
If you are really concerned about this, I would first consult my general practitioner and, dependant upon your feelings and the outcome of your discussion with your doctor, request a referral to an ent for consultation.
Remeber to tell your consultants everything you have included here, and anything else that you may remember.
