When working at the public library and before that, a bookstore, the colour of a book in question would often come up. Someone comes in and is looking for a book they read in childhood, or one they had browsed in the stacks and then a month later want to revisit it. A person once came in looking for a particular history book and remembered little else about it other than it was red. And we found it! That was fun.
A summer job I had ages ago was to take a small library that had been classified according to some cleverly invented colour coding and convert it into LC. After seeing the above photos I have half a mind to rearrange my bookshelves.