Color may be used on both the cover and the interior of your book. For color, most printers use a four-color (CMYK) printing process. Black-and-white and grayscale printing is considered a one-color process because the only actual color involved is black.
When considering the use of color, various factors come into play, including preferences, source images, and cost. You may wish to use color for your cover and/or interior, or you may have only grayscale images to use in your book. For many authors, however, the final decision will be based mainly on cost. As you may surmise, color printing is more expensive than grayscale printing. When it comes to covers, though, the increased cost for use of color is not usually as prohibitive as the cost of using color inside the book. Furthermore, using even one colored object inside your book will cost the same as using color throughout the entire book. For these reasons, many authors choose to use color on their covers and grayscale only throughout the interior.
NOTE: Use of color adds one more complication to the process. Colored images and text created with common computer applications are usually in RGB format. Additional steps must be taken to convert these colors to CMYK format for printing.