I think most people here are misguided in their advice of simply using whatever ink or ink color you feel like using. There's very much more to consider than what color to choose, because you're not corresponding with yourself. Correspondence involves you and the reader, so there should be some consideration for the other person. After all, you're writing to communicate with them, and you would want them to be amenable to what you're saying, rather than distracted or put off (or worst of all, insulted) by your ink choice. There are also times that your ink choice actually does matter, depending on the recipient and purpose of the letter.
So the ink I use to write to a Fortune 500 CEO seeking a donation for the non-profit where I volunteer will be decidedly different from the ink I use to write to my bestie who now lives five states over from me, and both of those will be different from what I send to my mother or to a professor.
For my bestie, the sky is the limit for color, although I try to make the choice at least medium in color saturation, because our eyes aren't getting any younger. Beyond that, she delights in getting any crazy color I can throw at her--Apache Sunset! Ooji Cherry! Diamine Pumpkin! De Atramentis Mahatma Gandhi! Pilot Iro Ku-Jaku! R&K Solferino! I can get away with this ink lunacy because we're still as silly about things like fun ink colors as we were when we first met, oh so long ago. That's why she's my bestie.
For my elderly mother, I don't have to be completely uptight, but because she's my mom, I don't feel comfortable being as free with her as I am with my best friend. That means muted colors like J Herbin Pousserie de Lune, Kobe Bordeaux, Maruzen Renga, Monteverde Jade Noir, or Diamine Chocolate Brown are perfect for correspondence with her. They're colors that convey respect and seriousness as befits my mother, while allowing me some individuality at the same time. For the holidays, though, she's like a kid, so she loves all my shimmer inks. Doesn't matter the shade. If they sparkle, she's delighted, although Emerald of Chivor is a perpetual favorite. She fell in love with it at first sight, so I usually have plenty of that in any holiday correspondence.
For most academic, business or other "let's be a little more serious" correspondence, I find that most of the blues, blacks and blue-blacks work perfectly for all occasions. You can push the envelope into the more muted colors like those I use with my mom in some environments. It depends, though. I've had situations or people with preferences that made it clear that it would have been completely unprofessional to use any colors other than blue or black. And I've had recipients, namely professors, who didn't care if I wrote in ketchup, as long as they could read what I'd written. You have to play any variations in this category on an individual basis.
For my most important business correspondence, I keep a bottle of Aurora black. It's a hardcore black that has little if any shading or color variation to it. And it looks C-O-R-P-O-R-A-T-E. It's scary corporate. Go look at my reply to Visvamitra's review of that ink for how scary I find it. But that it's so relentlessly corporate and button-down makes it exceedingly effective in numerous hyper-serious business applications. That Fortune 500 CEO? I got the donation we'd been trying to get for several months, because he not only appreciated the thoughtfulness of receiving a handwritten letter, but also because I hadn't insulted him by using a cutesie ink color. The no-nonsense black color conveyed my respect and the seriousness of my purpose in writing to him. Little things like that matter in that rarefied environment.
This isn't a slam against individuality. Like I said, I love to express myself as much as anyone else. But at the same time, I don't discount the bigger picture of how the needs or expectations of my recipient matter, too. Anything I can do to communicate with them more effectively only makes the correspondence better, and if that means reining in my ink choices a little or a lot to facilitate that communication, then it's a small price to pay.
Edited by Aquaria, 30 July 2018 - 15:23.