Color Theory: The Basics
Converting your color image files from RGB to CMYK is an important step in prepress preparation of digital files. Scanners and digital cameras create images using the three primary colors of light: Red, Green and Blue; an image is printed on a press, it typically uses four colors: Cyan, Magenta, Yellow and Black. Monitors use these same three colors to display images on screen. Like a scanner, a monitor generates pixels by transmitting light using the additive primaries, red, green, and blue.
Reflected light from a proof or printed piece results when ambient light bounces off the paper and is filtered by transparent inks sitting on top of the paper's white background. These two color models are so different that even expensive and well-calibrated monitors will not consistently predict the color of offset printing. For predictable results, make color evaluations using a printed proof, not your monitor.
Conversion and Color Shifts
While it seems easy, some things have to be considered. When an image is converted from RGB to CMYK, it moves between color spaces. Because of the nature of the two color spaces, not all colors in one are contained in the other. A gamut describes the range of different Colors that can be seen or reproduced. Notice the difference between offset printing's CMYK gamut compared to the gamut of a typical RGB monitor. When RGB images are converted to CMYK, there is some color shift to keep the image within a range of what is printable on press. The conversion to the smaller CMYK space tends to impact deep blues and greens hardest. The ink, the paper's brightness, and a number of press characteristics determine the actual range of color that we will be able to achieve.
Typically, RGB has a larger range of colors (or gamut) than CMYK. Some colors may be clipped or compressed as you convert from the larger gamut of RGB to the smaller gamut of CMYK. Colors not contained in a color space are said to be "out of gamut."
While it is important to convert from RGB to CMYK, do it only once because some colors are being thrown away, compressed and changed each time. Flip-flopping from RGB to CMYK and back to RGB and then to CMYK can noticeably degrade an image.
10 Tips for Great Color
1.Scan images in RGB, and save an unedited backup copy.
2. Many advanced filters only work with RGB images; apply them before converting.
3. Convert your edited images from RGB to CMYK just before the final save.
4. Create original art and illustration (especially with blocks of solid colors) in CMYK.
5. Make blends in CMYK.
6. Once you have converted an image to CMYK, do not convert it back to RGB. You will lose color quality and fidelity with every conversion.
7. Remember to convert images from stock photo houses or those on CD-ROM.
8. If your digital camera creates 16-bit color images, convert to 8-bit Image > Mode > 8 Bits/Channel , but only after converting to CMYK.
9. Record a Photoshop Action while converting an image and then use Batch File > Automate > Batch to convert and save a group of images automatically.
10. When saving a CMYK file, do not embed a color profile.
Be mindful of memory colors. Familiar objects such as a blue sky, a yellow banana, a red apple or green grass are called memory colors. We know what they should look like and when they don't, we notice it. Evaluate memory colors carefully when reviewing proofs.
1. Set your color preferences: Photoshop > Color Settings
2. Choose the CMYK setting based on paper and press type.
3. Convert a file from RGB to CMYK: Image > Mode > CMYK
4. Save files without a color profile.