Summary
Pilot's Iroshizuku inks are a line of fountain pen inks, and are considered Pilot's best inks, and among the best fountain pen inks out there. They're available in a wide variety of colors, each of which has a Japanese name. The currently available colors are:
Kon Peki - "Deep Azure", self-explanatory
Tsuki-Yo - "Moonlight", a blue-black
Take-Sumi - "Gray Black", self-explanatory
Shin-Ryoku - "Deep Green", self-explanatory
Syo-Ro - "Pine Tree Dew", blue-green, more blue than green
Fuyu-Syogun - "Old Man Winter", a deep, slightly-blue gray
Kiri-Same - "Autumn Shower", a lighter gray
Tsuyu-Kusa - "Asiatic Dayflower", a bright blue
Asa-Gao - "Morning Glory", a deep navy-blue
Ama-Iro - "Sky Blue", a light blue
Yama-Guri - "Wild Chestnut", a greenish brown-black
Kosumosu - "Cosmos", a reddish-pink
Chiku-Rin - "Bamboo Forest", a slightly-yellow green
Tsutsuji - "Azalea" (a flowering shrub), a bright pink
Yama-Budo - "Crimson Glory" (a flowering vine), a dark reddish-purple
Ku-Jaku - "Peacock", a deep turqoise
Momiji - "Autumn Leaves", a bright red
Fuyu-Gaki - "Persimmon" (an orange fruit), a bright reddish-orange
Murasaki-Shikibu - "Purple", a deep lavender
Trsukushi - (I couldn't find a translation for this one anywhere. Apparently it's both a Japanese name and also a restaurant. It also has something to do with Sushi.), a brown.
Ina-Ho - "Rice Ear", a golden-brown]
Shin-Kai - "Deep Sea", a muted blue
Asijisai - "Hydrangea", a deep lavendar-blue
Yu-Yake - "Sunset", a yellow-orange
Subjective Impressions
I have personally tried Tsuki-Yo, Fuyu-Syogun, Chiku-Rin, and Fuyu-Gaki, my personal favorite of which is the Fuyu-Gaki. The ink is water-based and writes like a dream; it flows well in almost all of my pens (with the exception of my flex-nibs and extra-fine nibs). It glides pretty well across the page, isn't too watery, and isn't super thick either. I've never had a problem with the ink separating but I always shake them up beforehand just to be extra safe.
If you're looking to get into the fountain pen world, they have ink sets in which you can get three rather small bottles of the ink in three different colors and try out each of the colors. It's great for trying out and finding a color you like. You can also buy small sample vials from fountain pen distributors such as Goulet Pens.