CMYK vs RGB
On the other hand, printed material displays color by absorbing varying colors of light that is shined on the sheet by outside light sources. Printing, then lays down combinations of inks that absorb light. Color can be represented by using an ink which has the desired color (called a spot color and usually described in the Pantone Matching System, PMS), or by building the color in the four color process model using four inks Cyan, Magenta, Yellow, and Black (CMYK - K is used for Black, and stands for "key"). These inks are called process inks.
Using four color process, a color is represented by a specific percentage of each of the four process colors.
There are a lot of complexities related to knowing exactly what color is described by a particular combination of RGB colors or process inks. This is discussed here.
Your eyes can see a wider range of colors than can be displayed on a standard RGB display device or printed in four color process. The range of colors that can be represented is called the gamut of the particular color model. For more detailed discussion, visit this Wikipedia page:
There are several different RGB gamuts, depending on the particular display or source device and how it is set up (sRGB, Adobe RGB, Pro RGB, etc). But in all cases, the gamut of RGB is larger than the gamut of four color process. That is, you can photograph, scan, and display colors which can not be printed exactly using four color process.
Parenthetically, this is partially addressed using six-color printing or printing with four color process plus selected Pantone colors. These are specialty printing processes and cost accordingly. We don't do these.
All of this is relevant if you're preparing a file to send in for book printing. Your original images are generated in RGB, and they need to be converted to CMYK at some point before printing.
If you're working with Microsoft products (Word, Publisher, Powerpoint), colors are represented as RGB. If you feed one of these programs a CMYK image, the program converts it to RGB. Since you want to avoid repeated conversions back and forth between the two color models, I recommend that you use only RGB images, and then do the conversion to CMYK at the end of the process, after everything has been made into PDF format.
If you're working with a real layout program such as InDesign, then you have a choice of which model to use. In this case I recommend that you convert all images to CMYK and use the CMYK model throughout.
Whichever road you travel (Microsoft or a more professional layout program) it is important not to use mixed models - that is with RGB and CMYK both in the file. This is especially true if you use any of the transparency features such as drop shadow. A mixed model with transparency is very likely to come out wrong once transparency has been flattened and files have been converted to CMYK.
If we get a file that's properly set up as RGB, then we can do the conversion to CMYK for you; it's easy, quick, and free. But my recommendation is for you to do the conversion, so you can see the effect of the conversion before you send your files to us. If your file has any colors that are out of the CMYK gamut, you can see what happens to them, and perhaps make adjustments before proceeding.
When you do a conversion, please use this ICC profile for the CMYK: GRACoL 2006 or GRACoL 2013. With this profile you should get a good color conversion for everything that is inside the CMYK gamut.
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