Hi Henk,
Not knowing enough yet about hynobiids, I just have to go along with what the shopkeepers say they are when it comes to identification, though I'm always wary they may be something different. I don't keep any myself, though I might raise some location-known H.tokoyoensis larvae this year for release into the wild.
Saw your very nice pics at Amphibia Web and also noticed there are no photos there for several hynobiid species. I could easily fill in those blanks as I've seen them being sold at shops here -- but would prefer that any photos I post at that site are first authenticated with regard to what species they are. In the meantime, I'll try to post some more pics of other hynobiids here as I get them.
By the way, regarding color variation, I've since heard the following from my Japanese "expert source" (not sure yet whether I can identify him here...and hopefully he'll eventually join the discussion in person!):
"Unlike lotic-breeding hynobiids (e.g., Hynobius kimurae, Onychodactylus japonicus), there are little geographic variations in the skin color in lentic-breeding hynobiids (e.g., H. nigrescens, H. lichenatus) because they have concealing coloration, though skin color variations are found in some species (e.g., H. nebulosus, Salamandrella keyserlingii)."
The symposium is this coming Saturday, after which I'll probably start a new thread for H.tokyoensis. There's a lot of info out there already on this species (sorry, almost all in Japanese!). In fact, I just printed out a 100+ page report the other day. Always thought it odd that there's so much on hynobiids but so little on C.pyrrhogaster, woefully neglected in most local amphibian surveys I've seen!