At some point black becomes gray enough that you'd have to call it "gray" instead of black, but it's debatable where the threshold is. And, as with everything on the paper, it depends on the pen. I used to use Aurora exclusively in the '90s, but have been into Noodler's Black for the last decade. Or Borealis if I specifically _don't_ want waterproofness. But, in pens wet enough to make Quink or Pelikan 4001 black look "black," I'll still use those. Grey inks are completely not my thing, though I had a bottle of Sheaffer Grey, and still have 29mL left out of a 30mL Herbin Gris Nuage bottle somewhere. Diamine Graphite and the discontinued Chesterfield Antique Raven are dark enough I call 'em black. Good comparison article: http://www.jetpens.c...n-pen-ink/pt/20
I did once send a letter in Noodler's Yellow. In a padded envelope. With a blue LED keychain light (makes yellow look black). To someone who I thought would enjoy the novelty.
I discovered that trick working at an ink cartridge refill store. We'd test cartridges to see if any nozzles were clogged. It was easy to see the test grid for magenta and cyan, but a blue light really helped to see if there were any gaps in the yellow pattern.
I once used Diamine Apple Glory for one side of an A5. It's dark enough to read, IMHO, but a whole letter of it could be annoying. It's best, I think, with a broader nib for a few lines in a greeting card. Anything lighter than that I'd only use for decorations, if at all.
Pink or magenta require discretion, as well. I have once each used: Noodler's Ottoman Rose, R&K Solferino, and Iroshizuku Yama Budo. But, no, as with black, I wouldn't send the same person frequent letters in such lower-contrast or eye-searing colors.
I have sent (and received) Bay State Blue letters and am OK with those. That _is_ pretty bright, depending on how cool your light source is (not bad with incandescent or 3000K LEDs), but plenty of contrast to be readable.