As common as it is, black ink is a tad formal and gives a funeral cast to whatever it is you are saying. Colored inks are growing in popularity. While red ink gives a confetti touch to whatever you have to say brown ink softens whatever you are writing so that it is not so serious.
Ink used to be made from Oak gall iron, called iron-gall ink. The Constitution of the United States was written in black iron-gall ink.
Though it went on black it aged into a polished brown. Iron-gall ink was too acid for fountain pens. I use brown ink to give my letters a vintage look by appearing as though they were written with iron-gall ink a century ago that aged into brown. Though it looks great on Crane's ecru stationery sheets, it goes well with any cream sheet.