Sometimes I get an ink and it exceeds all of my expectations. Everything clicks, and I love it immediately. That happened with J. Herbin's Vert de Gris. It was the last of 5 Herbin inks from my recent order that I opened and tested, as I thought "well, it's just a tealy gray, how special can it be?" I was wrong--it's very special! Vert de Gris, along with Bleu des Profoundeurs, are exceptional recent additions to the standard line-up of J. Herbin inks.
Most here are probably well-familiar with J. Herbin inks in one form or another--the brand has been around for a very long time and offers inks in all colors of the rainbow, even with shimmer. The standard line of J. Herbin inks has been known as safe and gentle to fountain pens, even vintage. Saturation tends to be lower (thus easy flushing), and the formulations are advertised to be pH-neutral, though whether all the colors are close to pH-neutral has been contested by some. In any event, I've never had any problems with J. Herbin inks from their standard non-shimmer line, and since I own a bunch of vintage pens, I tend to go for more gentle inks.
But gentle does not need to be boring! In fact, this ink is anything but boring. The interesting thing about it is how beautifully rich and matte it looks in high quantity (such as with a flex nib), and its beautiful hue in person. It looks especially good on ivory paper. Water resistance is very respectable--the tealy-blue components wash off leaving highly legible dark gray line, and water does not reduce writing to a smeary mess.
If I'm going to fault this ink in one thing, it's that on worse paper it's more feathering prone than some other inks. No problems with feathering on good fountain pen-friendly paper.