Are Your Colors Press Ready?
If you're working on an image that will be printed on a printing press and you select a color that's outside the range of what a CMYK press can reproduce, you'll get what's called a Gamut Warning right within Photoshop's Color Picker. This is just to let you know that the color you've chosen is outside the CMYK gamut. Just below the warning is a tiny color swatch showing you what the color you picked will really look like when printed in CMYK mode. To find out where that color resides within the Color Picker, click once directly on that tiny swatch and Photoshop will pick that color for you.