RGB, CMYK, LAB, what color mode should I use and when?
Digital cameras and scanners, by default, produce digital files in RGB color mode. It's a good idea to stay in this mode for general image-editing work. The same goes for making ink-jet prints. In fact for most consumer applications, RGB color is the mode of choice.
The only reason you'd convert a file to CMYK is if it's going to be printed using a 4-color printing process.(Web, offset, etc) Although changing color modes is easy to do, CMYK conversions can be tricky, especially when it comes to color accuracy. Nowadays, most commercial printers employ a computer graphics department to handle image preparation such as this. That way everyone gets exactly what they want. If you have the option, let the printer do the conversion for you.