Black: is often used on book covers to evoke a sense of crime, mystery, suspense, or danger. It's the symbol of the unknown or shadows. It inspires a sense of suspicion, and dark secrets that may be hidden within the pages. In some cases, black may also be used to convey a sense of loss so a black cover might work for a book about grief or one with a tragic ending. Ultimately, the use of black on a book cover can create a variety of different emotions, from excitement and anticipation to dread and fear.

White: is a light, neutral color associated with purity, innocence, honesty, and new beginnings.

Red: Red is often associated with danger, violence, bloodshed and passion. It’s no surprise, then, that this color is often used on book covers for thrillers and suspense novels. Red can also convey a sense of urgency, which may make it ideal for time-sensitive non-fiction books. Overall, red is a power color that can easily make your book stand out from the rest. The combination of red, black and pale skin (white) on the book above probably had a lot to do with the book's success. Not only that but the book clearly shows the rule of thirds applied.

Yellow: is associated with happiness, optimism, and sunshine. Not surprisingly, then, this color is often used on book covers for feel-good fiction and non-fiction titles. Children’s books also frequently include yellow on their covers as this color appeals to young readers.

Blue: is often seen as calming, serene, and trustworthy. It’s a popular choice for book covers in the self-help and self-improvement genres. Blue can also suggest intellectualism and wisdom, making it a good choice for history books, biographies and self help guides. 

In the example given here the colours chosen suggest optimism, trust and intellectualism. The book cover follows the rule of thirds and the focal image suggests cheerfulness

Green: most commonly associated with nature and peace. For this reason, green is often used on book covers for environmental non-fiction and nature writing. However, green can also suggest wealth and prosperity, making it a good choice for business books and financial guides. I've also seen Green as the predominate colour in dystopian fiction implying a wild savage environment.

The example here is the dystopian savage world type. 

Purple: is a luxurious color that has always denoted royalty and prestige. It’s also associated with books that are lush, delicious, bewildering, and magical. Purple is often used on covers for business books or books about success. If your book contains high-level advice or strategies, purple may help convey the gravity of its contents. Try adding an element of gold to highten the richness.

Pink: The color pink is often used on book covers as a way to evoke emotions such as romance, love, and happiness. It's often used in 'cosy mysteries.' The light hue of pink instantly sets the tone for a book. It can also help to attract readers who are looking for a particular type of story. Pink is often used on book covers that feature female protagonists, as it is seen as a more traditionally “feminine” color. This example shows a hardback edition of the children's classic. Probably a book worth looking for in second-hand bookshops.

Brown: Brown is often associated with traditional hardbacks without a dustjacket. it's the color of earth and, as such, can push a feeling of grounding or stability. It is also a very neutral color, which can make it appear safe or reliable. When used on a modern book cover, brown can convey a sense of dependability or wisdom. Abook about nature might use a brown cover to evoke the feeling of being surrounded by the beauty and serenity of the natural world.