There have been a number of "Bonesium" formulae, as Reaper adjusted the mix over time for different purposes and as they gained experience with the process and the material. There's not a very clear-cut line between which formulae are which, and I suspect Reaper hasn't really announced every adjustment of the formula.
I'll refer to Reaper's various plastics, from those first "original" Bones up to the newest variation, as "Bonesium", and I put "original" in quotes to refer to a collection of similar variations on the earlier versions of the material as marketed, on the assumption there might have been older variations that predated the stuff used in the first Kickstarter (there might be some prototype materials that might be considered even more "original" than the oldest Bones I know of!)
The "original" Bonesium we are familiar with from the first couple Kickstarters and the other early releases was very soft, very flexible, very bright white, and very soft in detail: you'll find a lot of very vague detail like very smooth and blank faces, a problem with flimsy "noodle-weapons", lopsided bases, and other odd problems. KS2 minis might have gotten the worst of this. This is partly a result of the version of the the "original" Bones formula used, maybe partly a result of the manufacturing and QA, and partly because the original sculpts were designed for more traditional metal minis, which hold fine detail better than early Bones material: later sculpting took those differences into account, and later formulae (especially Bones Black and Bones USA) held detail better. You'll find a few cases of old Bones models re-sculpted recently to take advantage of Reaper's experience since those early Bones were released (the Bones V and I think Bones IV Kickstarters included a handful of re-sculpts, and there are a few re-sculpts in the new Bones USA line.)
Illustration: A KS2 era "original" Bones Kickstarter figure: the traditional sculpting and
older Bones material could result in very soft details, such as this cowgirl's vague face.
Some later (post-KS1) "original" Bonesium miniatures in white might have had something added to the formula to make them a little stiffer than other Bones figures: I think that Reaper learned from the first Kickstarter that some figures did not handle the soft material very well - particularly those with slender ankles, or larger minis that need to support a lot of weight on thin legs - so a stiffer Bones material might have been introduced to help support larger figures like dragons and the like. Such figures would have been fine in metal, but the soft Bonesium proved to be a little different from metal in ways that were difficult to predict! For similar reasons, Reaper would have started sculpting figures specifically for the differences in Bones vs. metal as well at about the same time, to ensure that things like the legs and weapons were a little sturdier, and faces and other details were a little clearer... figures that were originally sculpted for metal would have started getting refreshed or all-new sculpts in their translation to Bones as an extension of this experience. By the third Kickstarter, detail would be a little sharper, and figures would be a little less flimsy, but this was still much the same material seen in the first Kickstarter. Whatever the case, the white "Bonesium" is some pretty forgiving and durable stuff: you could put on work boots and stomp on these white "original" Bones figures (please don't), and they won't break or even deform very much (they just sort of smoosh flat, and then return to their original shape), making them great for even the roughest imaginable tabletop gaming use... but if they are manufactured with any deformity, they don't really bend into their proper shape very well: if it has a "noodle sword" and is standing at a weird angle on its base, it's probably stuck that way!
Translucent/transparent/"clear" Bones figures - like ghosts and elementals - started appearing about KS2 and are, I believe, mostly (if not all) "original" Bonesium... if any of these have been produced in Bones Black material, even after 2019, I haven't heard of it (though I haven't seen or heard most, let alone all, of Reaper's announcements, videos, and so on - I can easily be surprised!) As near as I can tell, most of these translucent figures are a harder variation on "original" Bonesium, or at least seem to hold their shape much better than the KS1 version of the material, as I don't think I've seen any of these figures with the "noodle-weapon" problem or lopsided bases. (If it's not some property of the formula itself that provides these results, then maybe it's some other factor, like less delicate sculpting, or more careful handling or QA in the manufacturing?)
Bones Black minis will be a very stiff, tough formula, and will be grey... I'm told the material is brittle, but in my experience it's pretty tough stuff and resists breaking, cutting, and bending pretty strongly. Most Bones Black minis will be only slightly more or less human-sized characters: larger sculpts like dragons, giants, and the like, if I understand correctly, will generally not be Bones Black, even if they are grey like Bones Black; the technology dates back to 2019 (just before the fulfillment of the fourth Bones Kickstarter, with a few models in this material being released ahead of the Kickstarter), and I think almost all sculpts from before that year (aside from a handful of re-sculpts) are in original Bonesium. Bones Black seems to have been a bit more expensive to produce, and the minis cast in Bones Black seem to be a little more expensive by a couple dozen cents for each mini. Bones Black also apparently required a slightly different approach to sculpting, I'm guessing because the material is a lot harder and tougher, and thus (I expect) more difficult to remove from the molds, so it's not a simple matter to start making the old Bones sculpts in Bones Black: those old figures would mostly need to be re-sculpted specifically for Bones Black, and as a result, I expect that all but the most popular and/or most troublesome old Bones figures will simply continue to use some variation on "original" (soft) Bonesium, even if the figures are grey now. In my experience, Bones Black looks fantastic, and it's very durable and tough to bend or cut: I found the "original" Bonesium minis to be very easy to cut and otherwise modify, but Bones Black puts up quite a fight in this department: Bones Black bases are especially difficult and perilous to remove for re-basing purposes, for example! You could probably stomp on one of these, and damage your boot, without deforming the mini very much - a weapon MIGHT break under stress, but this stuff is tough! Still, it's Bonesium, and Bones Black seems to handle cyanoacryclase CA glue ("Wacky Gloo") just as well as the "original" Bonesium, mixing nicely with older Bones bits and with hard-plastic multi-part mini kit bits for things like weapon swaps.
Not all grey minis are Bones Black, though - some of the grey minis are a soft, flexible formula that isn't very different from the white Bones minis, as near as I can tell, except for being easier to see the detail on. I believe Reaper found their early white Bonesium to reflect and/or transmit light in such a way that they were just tough to see and tougher to photograph, and later mixes included different dyes which added subtle grey or beige tints to the mix, also (I've been told) hardening the plastic just a little as a side-effect. These grey and slightly beige Bones figures may look a little different from the KS1 and KS2 minis, but they are not Bones Black, and will be just about as soft and flexible as any other "original" Bonesium figures. For example, I've gotten some early goblins and a ghost recently which were newly produced in a grey version of "original" Bonesium, but appear to be identical in every other way to their KS1 era white "Bonesium" equivalents in detail, flexibility, price, etc.!
There was also a version of the "original" Bonesium, also in grey, that had something added to it to make it significantly stiffer than the usual character miniatures... this formula was mostly used starting about KS3 for things like buildings (the mausoleum and its cemetery fences, for example), and the arsenal sets, and (I believe) the plain harder plastic Bones bases. I think it might have also been used for a few larger multi-part figures as well: I seem to recall that a couple of the dragons might have had their wings cast in this material (the clockwork dragon, maybe?) This material might be a little more brittle than regular "original" Bonesium, but it holds its shape really well, so the arsenal kits never had the "noodle-weapon" problem, which would be why it was a great material for buildings with thin walls and the like. Unfortunately, I get the impression this material might have also been a little more difficult to work with: there were multiple problems that led to the arsenal kits never being released beyond the Kickstarters, such as packaging issues, and maybe scaling issues (the weapons were a little bigger than intended, I think), and I believe someone said that so many arsenal kits failed QA - they were just not economical for Reaper to deal with in their form (at least, that's what I gathered from the explanations I've heard.) I don't have any reason to think this version of Bones held detail any better than other "original" Bones, but it did hold its shape well, and seems to have been well-suited for flat, straight, slender subjects like walls, spikes, swords, and the like; I wouldn't call it exactly "brittle", but it will crack and break if you put it under (moderate) stress, compared to the very forgiving regular "original" Bonesium. It's tougher stuff, I think, than the hard plastic used by companies like Games Workshop for their plastic multi-part mini kits, but definitely the most breakable version of Bones that I've seen.
And, Reaper recently announced a third official major variant (or at least marketing label) on the Bones miniatures: "Bones USA". As near as I can tell (I haven't seen one of these minis in person yet), this is a (minor?) variation on Bones Black (CORRECTION: Reaper confirms this is a very different manufacturing process from both Bones Black and older Bones figures!): Bones USA is a hard, grey version of Bonesium that holds detail extremely well. According to Reaper: "Bones USA are thermoplastic injected models using state-of-the-art technology to achieve amazing results. Coming full circle on a promise we made to the backers of our first Kickstarter, we are now producing miniatures in our factory in Denton, Texas. How can you determine if a model is a Bones USA model? It says USA on the bottom of the base!" Bones USA figures so far seem to be a little more expensive than Bones Black, being about a US dollar more expensive than the "original" bones equivalent (probably due to the cost of American labor compared to Chinese labor and shipping/handling costs.) I haven't handled any of these minis yet for comparison to other Bones minis, but I expect that Bones USA will be very similar (if not identical) in handling to Bones Black, physically indistinguishable from Bones Black other than the "USA" on the base (CORRECTION: customers who have handled these have indicated that the figures do look, feel, and handle noticeably differently from older Bones products, and Reaper confirms the manufacturing process is different. Early impressions are that these do take paints, inks, etc. really well, do hold detail at least as well as Bones Black, can be either a little more flexible or brittle than Bones Black products. At this early stage of development are a bit less consistent than more mature Bones products in terms of quality, hardness, etc.; I expect this to change as Reaper dials in the best formula and manufacturing processes for this material.)
The difference between the older "original" Bonesium and Bones USA is striking - Reaper has come a long way in their sculpting for the process, and in the Bonesium material itself, to get very good detail in the results, as seen in this sample Bones USA digital re-sculpt of an old Bones figure originally designed for metal with traditional sculpting techniques, and produced in the first Kickstarter, in 2012:
Side-by-side comparison of a classic Bones figure, vs. Bones USA with updated digital sculpt. "Original" Bones version on left is from KS1, which is much the same traditional sculpt used for the metal version. I don't think the detail is bad at all on this one, it's one of the better minis from the first couple Kickstarters. On the right, the "Bones USA" figure, digitally re-sculpted in 2021: maybe the re-sculpt wasn't strictly necessary, but this updated version looks great and does help show off the evolution of Bones! This particular "Bones USA" resculpt is a bit bigger than the original, but other Bones USA models are apparently closer to the original Bones figures in scale.
So, there's a lot of "Bonesium" variants, and not all grey minis are Bones Black; most human-sized miniatures from 2019 on will be either Bones Black or the similar Bones USA figures, both in a hard grey plastic, and some (like the one above) will be re-sculpts of older Bones figures, but almost all Bones figures from before 2019 will be in the softer "original" Bonesium - including some grey plastic figures and some hard plastic "original" Bonesium bits and buildings that might be confused with Bones Black. Some "original" Bonesium products are a softer material than others, with variations based on Reaper's experience and adjustments to the formula for different applications. Most figures made after 2019 bigger than, say, an ogre will probably be in "original" Bonesium, no matter what colour they are, and starting in 2021, Reaper has introduced Bones USA figures, which seem to be a variation on Bones Black, but made in the USA.
Maybe the only easy "tip-off" that you probably have a Bones Black figure, aside from referring to the packaging or researching it, will be in how crisp the detail is, and how stiff and unyielding the plastic is: if it's grey but very flexible and the detail is very soft, it's probably "original" Bonesium. If it's white, it's "original" Bonesium. If it's translucent/transparent/clear, it's (probably?) "original" Bonesium, and if it's a larger sculpture in any colour it's probably "original" Bonesium. If it's grey, stiff plastic, and a part of an arsenal weapon sprue or a building, it's probably just a harder variant of "original" Bones. If it is grey and has "USA" on the base, it's "Bones USA" and (probably?) similar to/identical with "Bones Black". If the detail is clear and sharp, the plastic very stiff, and it's a roughly human-sized figure (including gnomes, goblins, halflings, and a few smaller ogres or larger orcs), it's more than likely Bones Black. If it is grey, roughly human-sized, was sculpted since 2019 (that is, it appeared in KS4 or later), and costs a little more than similarly-sized older Bones figures, it's (probably) Bones Black.
TL;DR: Just because a Bones figure is grey, won't necessarily mean it's Bones Black! Most grey, roughly human-sized minis sculpted since 2019 will (I believe) be Bones Black (or the newer and similar/identical Bones USA), but Reaper does continue making and selling "original" Bones figures made after 2019, including some grey ones, and including (generally larger) new sculpts.
(Time frames are approximate, and some of the above is speculation, educated guesses, and wild guessing - corrections and other insights from anyone who knows better, as always, are welcome, as are observations from other Bones fans!)