Creating color separations is a process that depends on a number of factors all relating to the source image. While it is definitely more of a science, than an "art", the process is complex enough and involves enough factors under the control of the graphic designer that good (or great!) results are reliably obtained only by designers who have a great deal of experience.  Separations are a time consuming and highly detailed process which is why you will often pay much higher prices for a Vector File as opposed to a JPG or even a PSD file of your artwork.  The price; however, is offset by the fact that once you have obtained a Vector file of your artwork, you can use this file to create any project that comes to mind.  Vector files can be easily converted to any other file type.  A knowledgeable designer will be able to use this file type to create everything from screen printing to signage and basically everything in between.