When should I convert color images to CMYK?

Scanned color images should stay in RGB mode if they are intended for screen viewing (web sites, CD-ROMS, TV graphics, motion picture compositing).

If you are preparing an image for professional offset printing, you should probably convert to CMYK. But do the conversion as late in the process as possible. Once an image is converted to CMYK, it will have a smaller color gamut (in fact you may have just thrown out about 25% of its original colors and you won't be able to get them back). And once you convert to CMYK, you should NOT convert back to RGB because the image will degrade. Besides, many Photoshop functions only work in RGB mode (i.e. image editing functions and filters).

A typical print workflow will involve saving a retouched layered archival file in RGB, then saving a copy before flattening the file and converting it to CMYK. This way you'll have a two versions available for any media.

If you are preparing an image for inkjet printing (i.e. a photograph intended to be framed), you might want to leave the image in RGB mode. Most inkjet printers convert RGB pixels to CYMK inks quite well (some inkjet printers use more than four inks and will print RGB images BETTER than CMYK images).

However, if you're using InDesign or any page layout program, ALL image elements should be in CMYK.