Brown fountain pen ink comes in a variety of shades. These are only a few of those available.
Brown Fountain Pen Ink Sample Set - The Goulet Pen Company
Best Brown Ink for Fountain Pens, EndlessPens
Macchiato Man
Brown Fountain Pen Ink - The Goulet Pen Company
Pelikan Souverän M400 Fountain Pen - Tortoiseshell Brown - Broad Nib
Alla Prima Still Life Painting Demo Damian Osborne
What Are the Best Browns, Drew ? - Q&A Slices
The Most Appealing Shades of Brown Color Ink - Ink Comparisons - The Fountain Pen Network
Monteverde Scotch Brown Ink Review — The Pen Addict
Favorite Brown Ink : r/fountainpens
Waterman Aztec And Absolute Browns Reviews And Comparison - Ink Comparisons - The Fountain Pen Network
Brown Ink — Mountain of Ink
Waterman Exclusive Fountain Pen - Brown Mottled, GP Trim, Fine 18k Nib (Near Mint, Works Well) - Peyton Street Pens
Brown Ink — Mountain of Ink