Four color process is the process used to reproduce color photographs in printing. As you may know, CMYK stands for Cyan, Magenta, Yellow and Black. "Okay" you say, "I understand the C for Cyan, M for Magenta, and Y for Yellow, but K for Black?" Actually it's very practical. B would get mixed up for blue. Or so I've been told. So, how does it work? If you remember when you were in grade school, they showed you how to mix red, blue and yellow to make other colors. Remember? Red and blue make purple, blue and yellow make green, etc. Well, that's how four color process works. As you can see in the pictures to your left, a color picture is separated into CMYK and when printed, it recreates the color picture. There are some limitations. In theory, you should be able to reproduce with only three colors - cyan, magenta and yellow. However, since we are working with ink that is not completely opaque, the shadow areas and black tend to look a little muddy. So black is added to give pictures the contrast it needs. Another limitation is that there is only a certain range of colors you can reproduce by four color process. This is called the gamut range. You will not be able to reproduce very intense or certain shades of colors. Photoshop has the ability to show you what your picture will look like in cmyk before you separate it. If you need to reproduce a color that is out of range, you will either have to use a spot color, print extra layers of process colors or use a color system like hexachrome which uses six colors for reproduction. Of course, all these options cost more money. You can only lay down one color of ink at a time. If you print a four color job on a one color press, you have to run the paper through the press four times, changing the color plate and ink for each color. Even a four color press can only lay down one color at a time. It is really just like stringing four presses together. You still have four color plates and four different inks.