In CMYK printing, registration black refers to 100% coverage in each of the four process colors: cyan (C), magenta (M), yellow (Y), and black (K). More generally, if a different mixture of colors is being used, registration black marks in each of the colorants (inks) used.
It is therefore not correct to simply define registration color as 100% of C,M,Y,K, since it will rather be 100% of whichever colors are used in reproduction.
It is a special purpose color, and as such is not generally used to print black text or grayscale graphics.
Instead, registration black is used for printing crop marks, or "registration marks". When proofs for each color are generated on separate pieces of film, use of registration black makes crop marks visible on all channels, providing a useful reference for alignment. A thin line printed in registration black can also be used to check whether the printing plates are lined up.
Read more about Registration Black: PostScript, PDF
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