Here is a simple technique for separating the colors so they may be printed on clear vellum, a.k.a. as "film positives". You will need a film positive for each color in your design. The film positives are used to put the image screen printing stencils. This technique is not for printing full color photos, in other words this is not for process color printing with halftone dots.
Open your design in Photoshop. If it exists in an Illustrator vector graphic format, export it to Photoshop and save it as a jpg or tif file.
Right away I will admit I'm still using Photoshop 7, as opposed to the latest version. Hey, I actually paid for it years back, and for my simple T-shirt printer needs it still works. So there you go!
From the tool menu choose the eye dropper icon and use it to sample the first color you'd like to separate, by putting the eye dropper over that color and clicking. From the "select" pull down menu choose color range. Move the fuzziness slider to 110 and click OK. The color is now selected in the design.
Open the channels pallet and go to the arrow in the upper right hand corner to open a pull down menu. Select "New Spot Channel". This opens the new spot channel dialog box. Click on color and select the same color that you are separating. Put "100%" in the solidity field. Name the color as you like. Click OK and your new spot color appears in the channels pallet.
Repeat this process for each color in your design until you have created a new spot channel for each. To print film positives, load your printer with clear vellum. Any channel that has the eye icon "on" will print. Be sure to indicate that you want to print registration marks in the print dialog box. By the way, it is a good idea to print out proofs of your seps on cheap paper first to check them.
If anyone has any questions about how to do this, zip me an email. It can be fussy the first couple times you try. And of course, if you need a quote on a job, I'm ready to help with that too!
Finally, there are times when I run into T-Shirt sep jobs that are beyond my capability. What to do in a situation like that? I highly recommend the services of Scott Fresener, the master of T-Shirt color separations.
Manx Media Custom Screen Printing