I've been running a 250W HPS lamp in a Cobrahead for a while now, and it appears that the glass is turning brown.  Do all lamps do this, or just certain brands and wattages?  Does the lamp itself cause this, or is it the heat not being able to vent like in a NEMA head.

The only reason for the outerbulb of HPS lamp to change color, is an arctube leaking.

Is there a "shadow" of the support frame on the glass ? (where the frame hides the arctube)

Is there a "shadow" of the support frame on the glass ? (where the frame hides the arctube)

No shadow, just the top of the outer envelope is a little brown.  It's a little worse on the PF400 400 Watt Sodium I have running.

My no name 70w HPS from a yardblaster developed a little halo of brown at the top of the lamp. It would light, but would cycle sometimes and not sometimes. Its demise was leaving it to sit over night and not letting it run. I almost recovered it by ovening it to raise the arc pressure and running it on an instant start to revive. I did get bright yellow flashes on instant start but no strike. the other reason was it had the top electrode (same part with the brown) detach.

I have some lanterns that have barely visible tint on the glass, bu they work.. And there is "shadow" of the support frame, where the frame hides the glass from plain sigtht of the arc tube

And most of those leaking lamps keep going for quite a while, cycling or not. They stop if the arc tube really blows and releases everything

Yes,have had this "smoking" or browning of glass on HPS lamps and --Vacuum tubes.Some transmitter tubes do this,too.Standard incandescent bulbs do it as well-emissions from the filaments.HID bulbs--very slight arc tube leakage-esp with ceramic arc tubes-they are very slightly porus.Gas molecules can leak out of them--or into them.That is one reason ceramic envelope power tubes and vacuum caps get "gassy" when not in use-when used -a "getter" element inside these absorbs the gas.HID bulbs also have a getter in them.It can only absorb so much gas.