By default youre working in RGB, that is, each color you see is a blend of red green and blue. For printing, it is impossible to use red green and blue inks because rgb is an additive color model which means that if you add all of the colors together, you get white. You can't do this with inks, obviously. Lets say for instance you wanted to get yellow. You can not use green, red, or blue, or any combinations of these colors to get yellow. But if you take yellow and add it to cyan you can get green, and so on. CMYK, which is what 99% of all home printers use, has a more limited amount of colors that it can express. The amount of color a colorspace can express is called GAMUT. Any color in your RGB image that falls out of the CMYK(printing) range is called OUT OF GAMUT. To see out of gamut colors in your image in photoshop, hit ctrl+shift+Y. To see how these colors will be expressed in CMYK, press ctrl+y.