Noodler's Lexington Gray Ink has a really beautiful shade of gray.
There's my writing sample. From the top, that's Bulletproof Black, followed by Lexington Gray, and finally a diluted mixture of Bulletproof Black.
Lexington Gray has a nice intensity to it. The colour temperature is cooler compared to the diluted Bulletproof Black. I find that with most of my other black inks, when I add water to them to make a gray, the gray would usually be of warmer colour temperature. I could never get a nice cool gray like the Lexington Gray.
The ink is water-based and flows really smoothly. It does not feature on paper and is waterproof when dry.
In the sketch above, I've also used graphite pencil just to compare the grays. Lexington gray's shade looks quite similar to graphite, except graphite is slightly more black.
The price on Amazon US is currently USD 12.50 not including shipping. That's a good deal for 90ml capacity.