Black for business, Blue for social correspondence, Red for correcting errors, Green for stocktaking
Today, I wouldn't use anything but black, blue-black, or conservative dark blue for business, official, or formal writing. The sky's the limit for personal use.
In my previous life as a college instructor I used red and green to make corrections on student papers
A notary once asked me not to sign documents with black ink, because it looks very much like a photocopy. So I avoid signing things in black
Many licenses, etc. must be signed in a colour other than black for legal purposes.
As an attorney I try to make sure to always sign in blue so that I can easily tell the difference between original documents and copies.
Why not black? The clerks have asked me to go to blue (as well as other attorneys) because copy machines are so good now that it is hard to tell the difference between original orders and copies that are to be attested