Cover styles are constantly changing as are the spectrum of colors used in those designs. Certain genres certainly use some colors more than others, but more and more the whole rainbow is being used. At the root of it is definitely marketing and attempting to draw in an audience, but do the colors being used really influence people to buy the books?
When looking at YA books, or even contemporary romance there appears to be a rapid increase in illustrated covers with bright happy colors. Of course these would draw the eye and perhaps intrigue someone enough to pick up the book and read what it’s about. That’s really what cover appeal is all about, being able to draw in a reader from something as small as a simple glance.
What colors are most popular? If you look at popular genres now I would say for the most part it is those bright, but not glaring colors. Certain genres of course have certain colors they use, for example a lot of thrillers and horror will use darker colors to set the mood of their subject matter.
So the real question is, do the colors on a cover influence the decision to buy the book? In my case, not really, but they will definitely make me check out a book. The color and overall cover of books are really a type of ad, and for that purpose they definitely do their job. It will be interesting to see how the industry further changes and modifies the covers of books to better attract their consumers.