Tail light bulbs turning gold color & burning out?? 6v System help!!!

[still1graphics]

Hi, my 53' Chevy is all original and still has the 6 volt system in it. I am running a 8 volt battery to help crank it better. Here's the problem, my tail lights keep turning a gold/bronze color from the inside out on the glass bulb and they burn out! I have all the dash lights, front park lights and front headlights all working w/ no issues, so why does my tail lights keep doing this??? Thanks for any help or advise... (the bulbs in the rear tail lights re 6v8w)