Color Separation: The art of detaching a color graphic or photo into single-color layers. For example, to print full-color photos with an offset printing press, one must first separate the photo into the four basic ink colors: cyan, magenta, yellow, and black (CMYK). Each single-color layer is then printed separately, one on top of the other, to give the impression of infinite colors. It is also called a Four Color Printing as it uses four standard colors to produce an infinite variety of colors.
This type of color separation, mixing three or four colors to produce an infinite variety of colors, is called process color separation. Another type of color separation, called spot color separation, is used to separate colors that are not to be mixed. In this case, each spot color is represented by its own ink, which is specially mixed. Spot colors are effective for highlighting text but they cannot be used to reproduce full-color images.
Spot Color Separations (vector art) work best with designs featuring solid colors or half tone screens. Spot Colors have cartoon like look and are not photo realistic. They are used for logos and images that have black outline.
Process Color Separation, as explain earlier, is an image that is made up of four main colors- CYMK or cyan, magenta, yellow and black. The image, if look carefully are actually made up of halftone dots.
Simulated Process is the most realistic process when printing photographic designs on both light and dark colored shirts. Simulated Process is not process colors of CYMK. They are actually halftone spot color images.
Simulated Process is a high end process using spot colors to simulate photographic images.