Iroshizuku has recently launched a series of limited edition inks, based on the Japanese Gods for luck. A clever marketing scheme, at 1.5 the normal retail price and colors that are not always easy to distinguish from the normal collection. Try to do a proper blind test between Murasaki-Shikibu and Jurosin for instance. I wouldn't bet on my ability to always tell these two apart without a paper chromatography.
But, arguably, Iroshizuku makes the best inks in the world, so let's give them some credit. And, truth be told, some colors are a welcome addition to the portfolio.
The color I do like best from these limited edition series is Hoteison. A very dark green (almost black). The ink behaves perfectly, flows smoothly, has a nice subtle shading and is a dream to write with in any fountain pen. Of course this ink deserves high quality paper.
The color is neutral enough for a business environment. No one would frown upon notes or signatures in a black green color. I'm not too sure about personal correspondence, I think it lacks warmth for it, or something that would really set it apart from other colors. Being neutral comes at a price. At the same time, I can easily think of a few subjects to write about that would really be suitable for this color.
It was hard to find comparable colors in my collection. Take-sumi by Iroshizuku is a black, Vert empire by J. Herbin is a darker green with low saturation but way lighter. Maybe Moss Green by Graf von Faber Castell comes close but is still considerably lighter.
N.B. Writing is on Original Crown Mill Vellum paper.