You may have heard about two of the main color modes used in design: RGB and CMYK. What do these mean? The easiest, and perhaps most important, thing to remember is that anything produced for the web should use the RGB color model, and anything made for print uses CMYK mode.
Computer monitors emit color as Red, Green, and Blue light, and use a mixing technology to produce other colors. Paper, on the other hand, absorbs or reflects light, so a different mixing system must be used for printing. Printers mix Cyan, Magenta, Yellow, and Key (Black) ink. These serve as filters, essentially, and subtract varying degrees of red, green, and blue from white light to produce other colors.
Both RGB and CMYK can produce almost any color, but the mixing processes are very different. This should be taken into consideration when choosing a mode in an editing program such as Photoshop. Printers will accept RGB files, but you might not end up with the color you expected.
When more light is added in RGB, it produces brighter colors, whereas adding more ink in CMYK results in darker hues. So if you achieved very vibrant colors in RGB (by adding light), this may result in a dull final product when printed. If you want more control over your printed design, it’s best to first convert the file to CMYK.