First of all, some colors complement one another well while some colors clash.
For example, blue and yellow usually provide nice contrast next to one another, but orange and red beside one another are hard to distinguish.
Text absolutely needs to contrast well with anything that is behind it. When text clashes with any part of the background, the result is illegible, especially in the thumbnail. For example, imagine navy blue text on a brown background.
Colors that don’t contrast well can appear beside one another in images, but not when one part of the image needs to stand out against an adjacent part. For example, if a girl is eating a red apple, the apple should clearly stand out, so there shouldn’t be colors that clash with red adjacent to the apple.
Two exceptions are color blends, like a gradient from pink to red, or accents. Blends and accents can have colors that don’t contrast near one another. A fiery picture may include blends of red and orange, for example.
Bear in mind that there are differences in hues, tints, shades, and tones. Not all reds look the same. So while red often contrasts well with black, there are many variations of red and black that don’t contrast well. A dark red doesn’t contrast well with black, for example. Will purple and pink contrast well? It all depends on which purple and which pink you’re talking about.
Another issue is how many colors to use. A good rule of thumb is to use three main colors. If the central image is a photo with several colors, the three-color rule might not seem feasible, but often the photo will have two or three main colors. If there is a main image, the other colors need to coordinate well with this.
Three colors won’t all contrast well with one another. The primary and secondary colors should provide excellent contrast, while the third color should be an accent that complements either the primary or secondary color. Different tints and shades of these three colors can be used when the design requires additional colors.
Black, white, and red can work well with one another. But, again, it all depends on which hues, tints, shades, and tones are used. Many other combinations can work well, if done right, like purple, yellow, and pink. There are several free design programs available online that help you choose sets of colors and see how well they work together.
How much of each color should you use? Another good rule of thumb is 60% for the primary, 30% for the secondary, and 10% for the accent.
It’s not just a matter of finding three colors that work well together.
For one, the use of color helps readers who are browsing through thumbnails find the types of books that they are looking for. Red is more likely to attract romance readers, for example. Pink is common among feminine books. A cover is ineffective when it attracts the wrong audience.
Browse the top-selling books similar to yours to see what color schemes are popular in that genre. This is what those readers are accustomed to seeing. While you’re there, see how many of those books followed these ‘rules.’
Color is not just aesthetic, it’s powerful. Colors evoke emotion.
For example, blue symbolizes trust, so many financial books feature a deep blue, while yellow is associated with happiness or intellect. Use colors that fit the content.
Colors can even have specific effects. When red is used against a background that it contrasts well with, it may help stand out and call attention to the book. Green is a relaxing color. Use blue for knowledge, white for simplicity, purple for luxury, gold for prestige, and navy blue for cheap.
Bright thumbnails tend to stand out better in search results. Yellows and oranges work well as highlighters against dark backgrounds.
Check that your color scheme agrees with the audience that you’re trying to attract. Don’t use pink or yellow for masculinity, for example.
Note that there is also a difference between the cmyk (cyan, magenta, yellow, black) color subtraction scheme used to print a cover and the rgb (reg, green, blue) color addition scheme used to produce an image on the monitor. If you’re designing a paperback cover, the printed cover will probably look significantly darker than what you see on the monitor, and colors that work well on the screen may not look well in print. Out-of-gamut colors are likely to look much different. Solid regions of one color may not look smooth. The best thing is to print a few tests before putting too much effort and commitment into the cover (keeping in mind that any single printing may suffer from possible variations, as 100 printed covers won’t all look identical).
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Chris McMullen, Author of A Detailed Guide to Self-Publishing with Amazon and Other Online Booksellers