Print true CMYK

Question

I'm trying to see what my local print shop's self-serve printer's cyan and magenta, and various shades of them look like. If I create a graphic in Adobe Illustrator that contains R=0,G=255,B=255; R=255,G=0,B=255; R=128,G=128,B=255 and send the PDF off to the printer, will I get actual prints of C=100,M=0,Y=0,K=0; C=0,M=100,Y=0,K=0; and C=50,M=50,Y=0,K=0, respectively? I understand that G=255,B=255 is outside the CMYK gamut, but will the driver 'push' it to become C=100,M=0,Y=0,K=0?

I have no control over the shop's drivers though. And being a self-serve printer, it's not controlled by RIP.

Answer 1

No. Even if you start out with known CMYK values, it is not guaranteed that the output will result in those exact values. In fact, they almost certainly will not. A properly calibrated printer will have been calibrated against the target stock, in which case the white value of the stock will be accounted for in the CMYK adjustments. The CMYK input values inform the output, as the rendering engine compensates for several different factors in an attempt to represent the intended result as closely as possible. There are ICC profiles, both embedded and set as overrides that affect the curves for CMYK values. It is extremely rare to get a 1:1 CMYK input to output from file to print. On top of that, you are starting out with RGB values, which do not correlate well. When software converts from RGB to CMYK, it must do so using an RGB color space profile, like Adobe 1998, and convert to a CMYK target space, like GRACol or SWOP. This choice is made for you by default, but you can probably change these options in your image software preferences. It's not as simple as Photoshop makes it seem at first impression from the Image\Mode... menu. There are many different RGB and CMYK profiles.

Answer 2

If it's a self-service, it may not print true CMYK, they might infact be fully digital in which case RGB is likely used, at most CMYK simulated. But who knows, there's so many different kinds of printing now.