applications render in RGB color space and Cinema4D is no exception.
That is fine for video or web application but its a big no-no
for printing as all print work (professional or non-professional)
is done using CMYK.
But have you ever tried taking an RGB render to Photoshop and
converting it to CMYK? All your colors get changed, right? And
have you ever sent a render to a client that aproved the image
and then, after seeing it printed, tells you that it is nothing
like what he approved?
shader allows colors to be defined in CMYK color space. Of course
the final render will come out in RGB but the colors will already
be (more or less), legaly, inside the CMYK gamut.