Look around a bank at the assortment of ink pens - look at any assortment of ink pens anywhere, actually - and two colors dominate: blue and black.
Whether endorsing checks or signing some other legal document, the signatory is expected to use one of the two colors, and there are reasons for the custom.
While banks frequently accept checks endorsed in inks of other colors, they may do a double-take and inspect the check more closely.
"We definitely prefer black ink for check endorsements," said Barrie Higginbotham, executive vice president for BancFirst. "It scans better. Everything is scanned, and the scanners don't pick up the light colors like pink and yellow."
Black has its advantages. It creates the highest contrast on a piece of white paper - the reason books are printed in black ink. But some institutions suggest using blue ink whenever possible. Several credit card companies request that blue ink be used on their applications to avoid challenges to authenticity.
Many attorneys and notaries public also like blue ink. Many states mandate blue, black or either for legal documents.
Blue ink stands out on a black application form, and is not reproduced by standard photocopy machines. Documents signed in black ink might be confused with their copies. Black ink remains the runaway color of choice even for computer printers, so it is easier to identify an "original" signed in blue ink.
"In some areas like lending, they will use blue ink," Higginbotham said.
Two popular colors worn by bank employees are blue and black, and that carries over to check endorsements. Banks advise that account holders not get too cutesy with color selection when applying signatures.
"Some other colors get through because we scan, but some reject," Higginbotham said. "If a check can't be read by the scanner, and we have the customer standing in front of us, we will ask the customer to trace over it with a dark color."
Red ink has the highest possibility of creating a delay in payment. Some fraud prevention technologies void checks signed in red.
The color red has long been stigmatized at banks - any overdrawn account is "in the red," after all. Bank inspectors used to circle suspicious endorsements with red ink.
Red didn't reproduce well on the old Xerox machines, and it remains problematic today, with many bank scanners using red laser lights that skip over red ink.
Other colors might also delay processing. Some high-speed imaging devices through which checks are scanned can have trouble with certain color inks, but never with blue or black.
For about 1,400 years, the ink of choice was iron gall ink or "registrar's ink." A purple-brown-black substance made from vegetable tannic acids and iron salts, iron gall ink turned black with time and was considered most suitable for record keeping. Many surviving medieval and Renaissance texts are penned or printed in it. In use since the fifth century, iron gall ink fell out of favor only about 60 years ago with the rise of chemically manufactured inks.