Brown fountain pen ink comes in a variety of shades. These are only a few of those available.
Pelikan Souverän M400 Fountain Pen - Tortoiseshell Brown - Broad Nib
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Pelikan Souveran M400 Fountain Pen in Tortoiseshell Brown - Goldspot Pens
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Brown Fountain Pen Ink - The Goulet Pen Company
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Pelikan Souverän M400 Fountain Pen - Tortoiseshell Brown - Broad Nib
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Brown Fountain Pen Ink - The Goulet Pen Company
Favorite Brown Ink : r/fountainpens