I have been using Seitz-Kreuznach Chestnut Brown in cartridge form in a Kaweco Sport with broad and extra-fine nibs.
Chestnut Brown has medium saturation, so it can look lighter or darker depending on the pen, but it is legible even from an extra-fine nib. It shades so nicely, no matter the nib I’ve tried.
Here is a snippet of Seitz-Kreuznach Chestnut Brown bracketed by a standard blue and black ink, for context.
As with all the Seitz-Kreuznach inks I’ve tried so far, Chestnut Brown behaves very well. It flows perfectly and cleans up easily. I saw a bit of feathering, but only on my terrible “everything feathers” copy paper. It has very little water resistance: it vanishes on Rhodia, but on lower-quality paper some dye survives even after soaking.
Here is paper towel chromatography showing the composition of Chestnut Brown.
Seitz-Kreuznach sells Chestnut Brown at a great price, with very reasonable shipping charges. Living outside the EU, I order from Seitz Global, which offers Seitz-Kreuznach ink in 100 ml glass bottles for $9.90 and in 20 ml glass bottles for $3.25. Short international cartridges are $2.19 for eight or $3.62 for 14. George Seitz kindly added this cartridge to an order I placed, so I could sample the ink, before they started selling the smaller bottles.