Recently a client sent me a screenshot of his print book cover and his ebook cover side by side, and asked me why he saw such a visible color shift between the two.
I explained to him that the ebook cover is in the RGB color space, but the printed book cover is in the CMYK color space. RGB is the color space used for on-screen images, while CMYK is the color space that printing devices can capture. As a basic explanation, the RGB images look brighter because they are being shown on a lit screen. It's hard to achieve the same brightness with ink on paper. As you can see in the chart below, CMYK captures a smaller range of color than RGB captures (and both capture fewer colors than our incredibly-designed eyes can really see!)
My client's cover designer had always sent his bright teal cover to him in RGB, and suddenly before printing, he saw the teal for the first time in CMYK and was surprised at the significant color shift.
If you don't convert your images to CMYK before sending them to the printer, the printer must convert them to CMYK before printing. It's no big deal for them to make the conversion, but depending on the colors you are printing, you may notice that the printed piece comes back looking duller than you expected if you only saw proofs on your screen.
The following two color spectrums help you to see which colors are hardest for CMYK to achieve. The duller quality of CMYK is instantly noticeable.
Whether you're picking a color for a book cover or a logo, it's good to consider whether that color will be achievable both in RGB and CMYK. If not, you might want to consider adjusting the ebook color a bit to make it easier to match in print. Or, you'll just have to get used to the slight difference in color between your ebook cover and your printed book cover.