For my new Pilot Custom 823 clear demonstrator, I wanted purple ink. I chose Waterman Purple, mainly because of the pure purple shade of it. Now, I'm wondering if I made the best choice, considering the two issues above. Anybody here had problems with flow and/or staining with this ink? I'm wondering if R&K Solferino or J. Herbin Violette Pensee would be better choices? I like those shades too.

Flow shouldn't be a problem. I don't know about staining.

I don't use purple ink, so I can't speak from experience with Waterman Purple. However, I have lots of experience with other colors, and I've found Waterman and Sheaffer Skrip to be the least likely to stain of all the inks on the market. I have not tried Herbin, but from what I have heard, it also belongs in this group.

I've been using Waterman Purple (2 years now) with my Cherry Blossom Opera Club (which is made in white and cherry resin) and I must say I've had no complaints:
* writes really smooth
* love the deep purple shade
* didn't have problems with staining - the occasional ink that would get trapped on the white rigdes of the screw cap and barrel wipes off no problem, no discolorations to the nib or converter alike. Yes, it will stain my fingers when I clean the pen, but even that comes off with a good soap rub

Oh, forgot to add that I also had no clogging issues at all - my pen writes right away and I don't really clean it all that often (in the past I've cleaned it 2x before longer storage - but now I'm constantly using this pen and ink again and it's all trouble free)

Currently in love with DI Imperial Purple. No staining, beautiful color.

I've got Noodler's purple in my Parker Vac. So far as I can tell, it doesn't stain, and i've been using this ink in it for a while now.

I don't know how Noodler's compares to, say Waterman's, but having also used waterman's purple, and having no staining with that one either, I'll say that they're both stain-safe.

Waterman Waterman Waterman ...

I've used the Violette Pensee before. Don't know about staining--I've only used it in a black & chrome Sensa Meridian, fine nib (it didn't stain that pen, but how could you really tell). The color was beautiful, and I NEVER had flow problems, even after leaving the pen stored horizontally on my desk for weeks at a time (I know, not advisable ). Always started immediately, no skipping, no nib creep, held up to a splash of water surprisingly well for a non-waterproof ink. (I have experienced all these problems with different ink in the same pen.)