Hi and thanks for the question,
It would be interesting knowing if your dad has any nasal congestion
symptoms. If its the case, your dad might have post nasal drip
resulting from chronic sinusitis
Yellowish discharge, dark brown are very suggestive of a bacterial infection.
A clinical evaluation by an ENT specialist
shall be required to exclude or diagnose sinusitis, pharyngitis or allergic rhinitis
Examination of the pus, culture and sensitivity tests might be necessary for an appropriate diagnosis and management. Depending on the clinical examination, other tests like sinus X rays with Hirtz and Blondeau incidences could be of much diagnostic help.
I suggest you consult an ENt specialist for management.
Thanks and best regards as I hope this helps,