I don't know how to mix or change a turquoise ink, except by diluting an existing one, but I agree it's also useful to discuss ready-bottled inks on this topic thread.
Larry T, that's a great comparison page of ready-bottled inks!
The new Sheaffer Skrip Turquoise is just a little bit darker or bluer (I don't know which) than the old Peacock, on Larry T's scan and in Clark's 2006
Sampler.
The closest-looking swabs in Clark's turquoise section, keeping in mind that all swabs are inexact samples and I don't have real training in painting, photography, printing or graphic design:
Aladine Lagoon Blue (close)
Diamine Turquoise, also Yard-O-Led Turquoise (close)
J Herbin Bleu Pervenche (lighter and bluer than Peacock?)
Levenger Blue Bahamas (close; "G"ood in water and sunlight resistance, relatively the toughest in this little list)
Mont Blanc Turquoise (lighter in Clark and in Larry T's scan, greener in the latter)
Private Reserve Daphne Blue (lighter)
Swisher Nantucket Blue (lighter)
Clark's "G"ood rating for water resistance, in my own experience with a few of the inks he's rated, seems to mean that the writing is still very legible after drops of water that are quickly blotted up.
From
the Turquoise comparison page at The Writing Desk, I would choose from this list (with the same disclaimer):
Diamine Aqua Blue
Diamine Turquoise (also Yard-O-Led Turquoise)
Roher & Klingner Blu Mare
Visconti Turquoise
The inexactness of swabs shows up in the differences between these lists.
(Edit) From the swabs on Pendemonium's website, I would add
Noodler's Dostoevsky (waterproof)
Noodler's Britannia's Blue Waves (waterproof)
(End edit)
Feel free to disagree, of course!