"Trade dress" refers to characteristics of the appearance of a product or its packaging or advertising that indicate the source of the product to potential buyers. Trade dress may include shapes, typography and even colors.
Most former British colonies use red, white and blue in their flags.Target likes red. But so do Coke, Staples and CVS. UPS like brown, as does Hershey--but Nestle uses non-chocolatey blue.
When people see a big, bright yellow paperback with a diagonal black band and a title in "reverse,"-- they think DUMMIES. Even if a reader doesn't regard herself as dumb, if she was successfully educated by one "dummies" book, there's a good chance she'll consider another. Even when subjects and audience may be diverse, it can be good to make the same type of books look similar.
[below] Books in the "Chicken Soup for the" series use the same ornate letter "C" that Campbell's uses on soup cans.
[below] Scott Prussing hopes that folks who were turned on by one of his vampire sex books will try another. The cover design and titles clearly indicate that the books are closely related.
[above/below] I doubt that any other book series can duplicate the success of "dummies" with another color. However, I am doing my best with purple on my books about publishing. I removed the "beach" logo from the front cover of newer books but retained the "Create Better Books . . ." tag line.
[below] As my publishing plans evolved and it became apparent that I would be producing a series of ebooks, I decided to give them a consistent look, with a comic-book theme and purple band at the bottom. I redesigned the previous books to go with the newer ones. I kept the tag line, but took the logo off the front cover and use it on the title page.
[above/below] My recent books that are not about publishing don't relate to each other or to anything else. Maybe they should. With ebooks, I don't have to think about hundreds or thousands of books sitting in a warehouse that won't relate to my other books.