Here's a small scale comparison of the three turquoise inks in my possession: Waterman South Sea Blue, Visconti aquamarine and Diamine turqoise.
I played around a bit with Pilot Parallel pens, can't say I'm really a fan yet, I guess I'll have to practice some more... Especially the broader ones are quite demanding when it comes to writing angles and such - not at all like my forgiving stubs and cursive italics!
If not viewed exactly next to each other the inks look very much alike, but there are subtle differences. South Sea Blue is the darkest ink with the most prominent sheen, whereas Vixconti blue is quite light, greener than the other ones and has nearly no sheen at all.
They're all reasonably well behaved - though I've got the feeling that all turquoise inks can a little reluctant flow wise which can lead to trouble when used in pens with the same predisposition.
I'm planning to do some more comparisons like this one and show you my inks.
Pens: Pilot Parallel Pens 2.4, 3.8 and 6.0 mm
Paper: Le Typographe Belgian Notebook, dot grid (review about Le Typographe)
Pictures also starring a Danitrio Genkai in kuro-tame nuri Urushi.
Do you like and use turquoise inks? Which one is your favorite?