If you are interested in creating your own covers, or if you are a part of selecting your cover in your published work, you need to understand something about color theory.
Color theory is the associations and impressions we get when confronted with a certain color or set of colors. Color matters.
A wheel and some hex
Your computer generates colors based on combinations of basic colors. These are written in RGB (red-green-blue) or hexadecimal. Once you know the code, you can replicate any color.
Using RGB or hex is particularly important as you replicate your colors and branding across multiple platforms. What looks like pure fire engine red on my monitor may appear more like brick or tomato to you. But at least with a uniform color code, I can get it right if I need to copy the red from your page or cover.
Imaging programs such as GIMP and Adobe InDesign both have color picker tools which look like eye droppers. Select the tool, click on the color you want to replicate, and the tool will grab the correct hex or RGB coded color.
How does color make us feel?
As with a lot of the marketing issues surrounding books and book covers, a lot of this will depend upon the buyer persona or demographic associated with the most sales of your genre. Let’s say you are a science fiction writer. Then a lot of your readership is probably going to skew male, although if you write LGBT science fiction, you may find more female readers in the mix. Either way, how do they feel about colors? Furthermore, if you mainly have an American readership, their associations with colors will to differ from if your ideal readers are Canadian or Swedish.