Relative and Absolute Colorimetric

Both Relative and Absolute Colorimetric shift only those colors that are not reproducible in CMYK, leaving the rest of the colors largely unchanged. The main difference between the two is that Relative Colorimetric makes sure that white in the original image will end up as white in CMYK mode. That's not overly important unless you're using an RGB working space that creates a white that is darker than what you can create in CMYK mode. This feature is mainly used in the Print dialog box when printing an image to a desktop color printer that can reproduce a brighter white than a printing press. In that situation the difference between relative and absolute is the difference between simulating the “whiteness” of the paper that will be used on the printing press (such as newsprint) or not. If your image doesn't contain too many overly vivid colors, you might find that Relative Colorimetric might not be a bad choice.

When I'm converting an image to CMYK, I'm not consciously thinking about what all these choices mean; instead, I'm just trying each one and looking at my image to see which choice produces the most pleasing result. Each choice has both advantages and disadvantages, and the only way to find the best setting is to experiment.