I had simular problem, EP's head lights seam to go like a smoked colour after a while.
Thought this was on the inside, so tried to take headlamp apart and found it really difficult. I think the older the lamp the harder to get apart.
I then when and used a soft abraisive metal polish like Autosol on the outside and the difference was out of this world as the fading or loss of colour what ever you want to call it was on the out side due to sun damage prob?
But i always use this now and it looks like a new lamp every lime.
This is what i do so intirely up to you mate but be carefull with what you polish it with as it can become reaaly dull looking and it could fail you on the MOT.
Used this for a few years now and no trouble, once again up to you which rout you take.
Steve.