There is a rule in graphic and web designing: RGB for Web and CMYK for Print
There are a few color models used to define colors. The two most important ones here are: RGB and CMY. The RGB color model is an additive model, which means that we can achieve white color by increasing the values of its three "ingredients" (Red, Green and Blue)
On the other hand, the CMY (or CMYK) model is a subtractive one. In this case going towards white color means decreasing values of each color and we achieve black color by increasing the values of its three “ingredients” (Cyan, Magenta and Yellow)
CMYK is a color mode used in color printing. CMYK stands for Cyan, Magenta, Yellow and Key (black). In CMYK mode magenta + yellow produces red, cyan + yellow makes green, magenta + cyan generates blue and magenta, cyan and yellow on a white background produces black.
CMYK vs. RGB
CMYK, or four-color printing, generates a good final printout with excellent contrast. But the color seen on the screen may be different from the one that comes in the final print. This is because the computer screen follows a RGB system, and printers follow a CMYK system.
Why we can not use same coloring system for computer screen and printing? On one hand where computer screen release the light, a paper absorbs the light. A computer screen and a paper reflects colors on different wavelengths of light and that is why sometimes colors in same artwork displayed on a screen may not match with the colors in printouts. Even if we change the color mode of any artwork from RGB to CMYK using some software we can notice the differences in colors.
RGB to CMYK (for Printing)
Mostly scanners, digital cameras, and video capture systems save files as RGB and we work on artwork to correct levels or balancing or anything else before giving it for printing. Do you work on RGB and after you modify it, you switch to CMYK to print?; or you switch to CMYK directly and you modify it later?
When start with RGB image I prefer to do all or most of image editing in RGB itself as some editing procedures are not available in CMYK. Converting to CMYK is the last step only when file is being sent to a print house. Conversion of RGB files to CMYK can be done in many ways and the method is different from software to software.
Most of the printers prefer your files be delivered in CMYK format. Some may accept RGB with ICC profiles attached, as this allows the printer to use color management methods when converting to CMYK.
If you are printing from your home or office printer, you can use RGB or CMYK. Try both and see which you prefer.
Is converting images to CMYK important?
Yes! It's very important if you are working on a print document. Commercial printers mostly accept RGB images without question but using images with a RBG color space can seriously screw up your image. Printers who accept RBG images auto-convert images to CMYK without checking the outcome and they do not bother to adjust levels, balancing etc.
Make sure you convert images to CMYK yourself, so you can do adjustments on the CMYK outcome if needed and keep 100% control over image quality.
RGB: Good for computer/digital use. Bad and not usable for printing
CMYK: Good for offset and digital printing. Bad and not interpreted correctly on the computer.