Buddy:
     Unless there are local factors to take into consideration, the only cause
of brown comb I know of is that caused by brood being raised in the comb prior
to the honey being stored there.  The comb would also have a toughness not
found in comb which has never had brood.  Which I think is due to the remnant
larval casings.  Brood comb which has been in a hive more than 3 or 4 years
has a dark brown color.  I don't mean to gross you out, but it is my best
guess.
 
Best Regards,
Gordon