Are bronze colored carbide inserts actually worth more than the plain gray ones?

That bronze color is almost always a titanium nitride coating, which is one of the oldest and most common coatings out there. It doesn't automatically mean premium grade, but it usually means the insert was meant for higher speeds or tougher material than the plain gray uncoated ones your shop buys.

I got handed a similar mystery batch a few years back when a retired guy's tooling got donated to our shop. Some of them were fantastic, some were chipped or dull from sitting around, so I'd eyeball every edge under good light before committing one to a finish pass. A magnifying glass helps. If the edge looks sharp and the coating is intact, just run them like anything else. On the mild steel debate, my experience is somewhere in the middle. For everyday low carbon stuff the coated ones do last noticeably longer at the same feed and speed, but the difference in cost per part is small either way if you already own them. Where they really earn thier keep is interrupted cuts, harder material, or when you can't stop to index as often. I wouldn't save them for special occasions, honestly. Inserts age in the case too, coatings can flake, and a free insert that sits in a drawer for five years is worth less than one that made chips today. Run them on your normal work and if they cut clean, you came out ahead.

I'd push back a little on reading too much into the color. I inherited a drawer of gold-looking inserts when our old lead guy cleaned out his locker, and half of them turned out to be fine and half were junk. The coating tells you very little on its own. What actually changed how they cut for me was the chipbreaker shape and the nose radius, and you can't tell any of that from a bronze shine.

My advice would be the opposite of saving them for a special day. Mystery stock ages badly. Edges get nicked in a drawer, and some of that old coated stuff has a substrate that doesn't hold up the way modern grades do. If you hoard them for the tough jobs, you might burn a workpiece when one lets go mid-cut. I burned thru my inherited box on routine jobs first, kept the ones that performed, and tossed anything that chipped early. On the mild steel debate, both camps at your shop are kind of right. Coatings shine at higher speeds and in sticky material, but on plain mild steel at the speeds most manual or older CNC work runs at, the difference is modest. I'd run one in the same part you normally do and compare edge wear against your gray inserts at the end of the day. That tells you more than any color ever will.

Check the face of the insert itself, not just the packaging. A lot of them have the grade stamped right on there in tiny letters, sometimes on the underside too. I've ID'd mystery inserts that way plenty of times and just looked the number up.

Also I wouldn't baby them, but I'd run one on some scrap or a non-critical part first. If it chatters or chips wierd you're only out a few minutes. If it cuts nice, congrats, you've got free inserts for months.

Agree with the guy who said to look for the stamp on the insert itself. I dug through a coffee can of loose ones our shop got at an auction and found the grade etched on the underside of a few of them, just had to tilt them under the shop light to catch it. Looked the numbers up online and turned out half of them were meant for cast iron, which explained why they were garbage on the aluminum I tried first. Also second the run-them-on-scrap approach, I test one edge on a chunk of cutoff before I trust anything mystery like that on a real job.

Not buying the bit about inserts going bad in the drawer. Carbide sitting in a case for years is basically the same carbide it was the day it was boxed up. Coatings don't just flake off on a shelf, they fail from heat and cutting forces. The real reason to test one on scrap first is that you don't know how the previous owner treated them, dropped in a drawer loose and rattling around is how edges get dinged. I've run mystery inserts that sat forgotten for a decade and cut just as good as new ones.

Update: tried the trick about tilting them under the shop light and it worked, kind of. A few of the bronze ones do have numbers etched on the underside, but most are too worn down to read. Only got one clean stamp so far. Is one readable insert enough to guess the rest of the box matches, or should I just run one on scrap like someone suggested and see how it holds up?