Color is tricky at the best of times. If you want exact color management (such as a particular shade of blue for a university logo) you really should use something called spot color. Spot colors are specially mixed ink colors. Like the paint you purchase for your walls, the spot colors, or PMS colors (PMS stands for Pantone Matching System), are mixed according to pre-determined recipes. Each color in the PMS spectrum has an assigned number. When a client picks a number, the ink is mixed according to that recipe. It's usually more expensive than process color printing.
Process color is the most common choice for book covers, particularly self-publishers. Process colors are commonly known as CMYK colors. C (Cyan) M (Magenta) Y (Yellow) and K (Black) are the four colors of ink that are mixed together to come up with the colors you see on your book cover. With digital printing (also called print-on-demand or POD) it's usually necessary to pay for use of all four colors. If you're printing offset, then you might be able to save money by using only one or two of these colors. The most common choice would be black (since your bar code and interior pages will probably need to be black, anyway) and either one of Cyan, Magenta or Yellow.
It's important not to confuse what you see visually as only one color, or four colors, or 10 colors, with what the printer will consider to be one, two, three or four colors. Your cover may have a solid pink background with black text, but still be a four color cover. That is because all four colors, CMY and K were used to create your particular shade of pink or black. I'll illustrate this using Cyan, which you can view at: http://www.bookcoverexpress.com/colors.jpg
Now it gets more confusing. Aside from PMS colors and CMYK colors for ink, computer screens can only read and show you something called RGB colors. The RGB stands for Red, Green and Blue. While CMYK colors absorb light, RGB colors reflect light. What this means is that in the CMYK gamut, absence of all color (C at 0%, M at 0%, Y at 0% and K at 0%) will produce white (also called reverse). That is why CMYK colors are called Subtractive - the more color you subtract, the whiter the color. However, in RGB it's the opposite. The more color you add, the more white you have. RGB colors are called Additive. The white on your computer screen is actually 255 Red, 255 Green and 255 Blue. If you add into the factor that computers are also being lit up from behind, monitors vary, screen settings and operating systems and software varies between programs and between user settings, which anyone can change, you can understand why trying to judge what a color will look like in print on a screen can be tricky.
The only way to ever know for sure is to either go into the print shop and choose a color from their color swatches, or wait until the printer sends a print proof. Even the above color sample I pasted in is actually only an RGB rendition of the CMYK colors. And yes, even the CMYK in PhotoShop is only a close, RGB representation of the CMYK colors.
To be honest, it's rarely an issue. The colors are pretty close and most experienced designers and printers will be able to help. It's only in areas where you need to have an exact match, such as the university logo color I mentioned earlier, that you may want to get more hands-on with color management and discuss the issue with your printer. Your printer will probably be able to supply you with the CMYK code or PMS number that you need and you can give that to your designer.
There are things to watch out for, though, especially if you're switching from the RGB color gamut to CMYK "on the fly." Industry-standard programs such as those produced by Adobe and Quark are fine, but I've seen some people actually create covers in programs developed by Microsoft. Since these programs only work in RGB, color management is an issue. Some shades of blue, green and pink are particularly difficult.
At this link: http://www.bookcoverexpress.com/rgbpink.jpg you
will see an RGB repesentation of what can happen when proper color management is not employed. Although this is dramatic, it is pretty much what happens with this particular shade of pink. CMYK colors are often more subdued than their RGB counterparts. The CMYK color range is smaller, plus the absorption of light really impacts things.
For more information on colors check out these sites:
Cathi Stevenson is a former newspaper writer and editor who has more than 2,000 published articles to her credit. In publishing since 1981, she opened her own book cover design company in 2000 and since then BookCoverExpress.com has created more than 650 book covers for independent publishers and presses of all sizes. Read more of Cathi's articles about publishing at: http://www.thoughtblog.net