What is color separation for screen printing logos on a t-shirt.

The process of dividing the image into two or more colors is called color separating, and the film from which the plates are created are called the separations.

Previewing separations on your monitor lets you preview spot color objects in your document, and check the following:

Rich black Previewing separations lets you identify areas that will print as rich black, or process black (K) ink mixed with color inks for increased opacity and richer color.
Overprinting You can preview how blending, transparency, and overprinting will appear in color-separated output. You can also see overprinting effects when you output to a composite printing device.

Print color separations:

Choose File > Print.
Select a printer and PPD file. To print to a file instead of a printer, select Adobe PostScript® File or Adobe PDF.
Select Output on the left side of the Print dialog box.
For Mode, select either Separations (Host-Based) or In‑RIP Separations.
Specify an emulsion, image exposure, and printer resolution for the separations.
Set options for the color plates you want to separate
Set additional options in the Print dialog box.
Click Print