(Sarah Read is an author, editor, yarn artist, and pen/paper/ink addict. You can find more about her at her website and on Twitter.)
The Callifolio inks from L'artisan Pastellier are a line of non-toxic, non-corrosive, gentle inks made with natural pigments. Olivastre is one of the most saturated colors of the bunch, and the second most fun to say (after Andrinople, of course).
The inks come in 40ml bottles, or 50ml refill pouches. The bottles are interesting. They're the triangular wedge ones that can form a circle if you collect enough of them (or so I hear--it's a theory I'd like to test, as soon as possible). The pouches will only work if you have another container you can pour the ink into, as a pen can't be filled from the pouch itself. But I love the idea of refill pouches--I wish more ink companies would adopt that practice.
The formula is gentle enough that it can be used safely in vintage pens, and the colors can be mixed. I'm anxious to collect a few more colors so I can play mad scientist and concoct my own custom hues of this nice ink. I think that property gives this ink more potential as an art medium, beyond its standard pen-fill purpose.