When the market's best and cheap book cover designers UK create book covers they are required to keep a few points into consideration.
First, the cover of the book should represent the theme and mood of the book.
Add vibrant colors to represent a happy, romantic or lighthearted comedy mood, and use dark contrasting colors to represent thriller, horror, crime or other dark genre of the book.
Use font styles that are uncommon yet easy to read. Many book cover designers go for typography to add text to the book covers.