I don't think it's a colour variant, more so a differing colour pattern depending on their region of origin. Keep in mind I haven't researched this, I believe the Severums with a lot of blue around their face are from the Rio Negro, and Turquoise are the plain bodied Severum that sometimes develop red spangles through their body which are from another tributary.
I've got 2 Severums, both are very different, this might give you an idea.
Sub-Adult Severum that's possibly a Rio Negro. These are Heros efasciatus anyway but true
H. severus are quite rare in the hobby. Old photo because he's very skittish (still), February 2.
Bad photo of a Female Turquoise Severum.
Edit:
Average photo I took of a mates Turquoise Severum.
I just call them all Green Severum, they're not a different species at all, nor do I think they're even a colour morph. Anyone that's been collecting Severums able to enlighten us on this?
