I think you're jumping the conclusion that the darker the slurry to less garnets it contains. It's true to some extent, but I don't think it's the only contributing factor to the slurry color. Steel forms several different oxides. The main oxide we see on a hone would be ferrous ferric oxide (Fe3O4), a.k.a. magnetite. It's a type of oxide that forms when iron oxidizes under water.
Other iron oxides may have different colors. We all know the red variant, ferric oxide (Fe2O3) a.k.a hematite.
I'm not sure if an how the other components of steel influence the oxidation process, but I can imagine so.
I too have noticed that different steels tend to cause a different shade of slurry. That's why I always use the same Double Arrow razor to estimate the speed of a Coticule. All that is about to change, when the abrasion tester arrives that the good Ralfson has build to my instructions. It's a device that should allow the objective measurement of abrasive speed.
At any rate, a Coticule is not abraded by the steel, but by its own garnets that are already present in the slurry. That's the big reason why the Belgian Coticule is such a good honing stone. There are more rocks in the world that contain spessartine garnets, even of the correct size, but the binding element must be of the right hardness, to allow a very slow, but steady stream of new garnets being released. If you would transfer slurry to a glass surface, and hone on that, you'd notice that that slurry looses its cutting power after a short time.
It's possible that the released phylosilicates (the binding element in Coticule rock) react chemically with one of the components of the steel (e.g. Chrome, Molybdenum, Manganese, ...) have an influence on the slurry color. I think that's more likely the cause why you see a different shade in your slurry.
PS. It is a public secret that many tool and knife manufacturers overrate the HRC values of their tools. I've come across lab test that measured 58HRC on knives that were advertised as 62 HRC. Just to say that we must approach these claims with caution.
Kind regards,
Bart.