A deep dark brown ink, inspired by the color of "wild chestnut" by my favorite brand, Iroshizuku. It is subtle, it doesn't draw attention but it's absolutely beautiful and sophisticated. Perfect for both personal work notes in the office and personal correspondence.
Being Iroshizuku, it goes without saying that the ink is very well behaved. It performs excellent in any fountain pen I have tried it in, it is well lubricated, has a beautiful shading and drying times are around average. Water resistance is above average, an occasional drop of fluid won't ruin your writings completely. On decent paper (and this ink deserves that!) there is no feathering or show-through.
Most brown or brownish color I have are less subtle. It wasn't so easy to find two colors that at least a little bit close. Herbin's Cacao du Bresil and Caroube de Chypre (with gold shimmer) are dark browns too but significantly different.
Yama-guri is a welcome addition to my ink collection. Definitely an ink to use for a few days every now and then.