For one color designs it is not hard to separate or re-color that color and to make screens. For multi-color designs, as long as they are spot colors, it is also not so hard to separate those colors even if it will take longer than one color designs.
But for 4 color process(CMYK) or simulated process for full color designs, color separation requires a lot of experience. Just a little more or less of certain colors can make final print look very different for the worse in many cases. Getting the best full color separation is a time-consuming process and a product of many trial and error experience. This is why many screen printers use automated color separation software for commercial use. With this softwares you can save a lot of time in color separation which is very important in tight production schedule. We use UltraSeps for color separation and recommend to any printers because the quality is great and it is easy to use.