If another color is your goal, don't forget that you can mix Noodler's bulletproof inks.
If another brand is your goal, you'll have to look for an iron gall ink with brown color. Rohrer & Klingner has two iron galls, "Scabiosa" and "Salix," neither of which are brown but might produce brown in a mix with a non-waterproof color. You can see them
here on Michael Richter's website. As Michael says, iron gall inks require wetter-writing pens. Also, iron galls aren't good to leave in a pen for a long time.
Later edit: Diamine Registrar ink is a blue-black iron gall, also perhaps mixable with washable colors to produce brown. Mont Blanc also sells an iron gall; see the recently added-to thread on that.
Have fun!