I agree with You that my conclusion of an AL-31FN is not 100% for sure, but my argument - against any WS-10XYZ or even WS-15 - is based on several assumptions and - honestly - the argument, "silver" or now "dark colour" alone is not conclusive if all other physical details are different to any WS-10 we know but more akin to all AL_31 we know, ... besides the different colour; to admit, that's indeed a mystery.
But IMO it is much more likely that it is a specialised AL-31FN with some sort of modernised nozzle since the number of pedals, the afterburner-/flameholder structure is nearly identical, even more the typical screaming sound and flame are (very) similar, whereas all WS-10 so far have a different number of pedals, the typical inner structure of the WS-10 is even completely missing.
Again I only can say what's IMO the most likely engine based on several different factors ... but "a different coulour" alone so it MUST be a Chinese engine, simply does not makes sense.
Deino