True, but the real question is, “why won’t the new parts color like the old ones?” The answer is the difference in materials.
There's an art to this. Forgetting the color for a second, the reason that parts were case hardened was to end up with a part that was wear resistant but not brittle. Case hardening hardened just the surface of the part (making it wear resistant) but left the interior of the part soft and tough (not brittle). It gave manufacturers the best of both worlds - tough parts that resisted breakage and wear. Gears and gun parts were ideal candidates for this process.
To case-harden steel, you start with a low-carbon, low-alloy steel. Normally, such steels will not harden with heat treating because the carbon content is too low. Carbon is introduced into the surface (only) of the part by packing the part in with some carbon-bearing material (in the old days they used a combination of charcoal, animal hide, and charred bone meal). The packed part was placed inside a (more or less) airtight container to help keep the process oxygen-free, and placed in a furnace.
When the whole mess was heated to the proper temperature and left for a while, carbon from the packing material migrated to the part, changing the surface of the part (to a depth of a few thousandths of an inch) into high carbon steel.
Here's where the hardening part comes in. Simply adding the carbon does not automatically harden the part. In order to make the high carbon portion of the part hard, it needs to be rapidly cooled. After the part has been in the furnace long enough for the surface to become high carbon, the part is suddenly quenched (cooled) - usually by dunking the part in a barrel of water.
The sudden cooling hardens only the high carbon portion of the part. The quench has no effect on the low carbon steel.
You end up with a part that is hard, wear-resistant, and somewhat brittle on the outside, but tough, resilient, and soft on the inside. That's why do-it-yourself trigger jobs on case-hardened parts can end up going very badly - it's too easy to grind/file right through the hardened portion exposing the soft steel beneath. Most case hardening leaves the part a sort of grey-ish color.
Color case hardening is slightly different. To get the pretty colors, there were some extra steps and changes to the process. First of all, the parts that were to be color case hardened were polished and cleaned prior to going into the furnace (just like when you chrome plate today, surface finish is key). Animal hides and bone (no charcoal) were usually used as the carbon-bearing packing. The quenching process was a bit different too. Manufacturers found that the colors were enhanced by adding nitrogen to the quenching bath (very early on they just added gunpowder until someone figured out that it was the potassium nitrate that brought out the color. I know a heat treater that adds ammonium nitrate to the quench with good results). They also found that the colors were improved by agitating the bath by bubbling air through it. Each manufacturer's process was sligthly different and probably a highly guarded secret.
You can still have parts color case hardened today. I work with a heat treater that will do it. There are even do-it-yourself kits that let you case harden with a torch. The reason you do not see as much of it today is that color case hardening is more expensive than other methods.
Case hardening became unnecessary when steels improved. Deep-hardening steels like A2 can be tempered and cryogenically treated to be even tougher and more wear resistant than a case hardened part. Some of the so-called "superalloys" are even more amazing. There are crucible powedered metal tool steels like CPM10V and CPM15V that are incredibly hard, tough enough to be used to make high-speed stamping dies, and so wear resistant that they will wear out a silicon carbide grinding wheel (diamond is the only thing that will cut it). None of them look as nice as a good color case hardened part though.