CMYK values don't match RGB values

[Kander]

Hi there..

I'm super new to designing and started using A.Publisher a few weeks ago. I've been designing a logo in RGB in Inkscape & using APUB to do all my file conversions especially for stuff I need in CMYK. I'm aware that CMYK colors will usually come out differently from RGB when it's sent to printers. 

Anway, I decided to just do a quick check on my colors and noticed that the CMYK values that's been generated by APUB don't match the RGB color's supposed CMYK color.. example. I have a color #134A84 which https://www.htmlcsscolor.com/hex/134A84 tells me should give me a CMYK value of 86/44/0/48 but in APUB, the CMYK value showing is 98/79/22/7 instead. When I plugged in the value of 86/44/0/48, I obviously got a different color and the RGB hex became #004B79. 

So I guess my question is, does APUB convert our RGB colors into CMYK to match what I actually see in RGB or is this a bug? This is a concern for me because I'm working on this for a client and want to make sure I deliver the right stuff. 

Thanks in advance!  

Edited July 20, 2019 by Kander
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[mac_heibu]

You can get hundreds of correct CMYK values when converting from RGB. Why?

You could(!) calculate from RGB to CMY mathematically, because this is basically a conversion from additive colours to subtractive colours. But this is not the way, it works for printing. CMY colours never will able to reproduce a „real“ black. So, for printing reasons, „K“ (= black) is added as an additional (fourth) printing tint. So CMY becomes CMYK. The consequence: An RGB color suddenly can have literally tons of CMYK equivalents. (In mathematics, you would say, the system is „overdetermined“.)

The way, how RGB is converted to CMYK depends on the printing process (speed/specs of the printing press, kind of „paper“, kind of printing tint, …, …) The precise way, how a certain RGB value is converted, is determined by the applied RGB profile of the source (image, for example) and the CMYK profile, into which you want the RGB source to be converted to.

For example R=0, G=0, B=0 will convert to C=88, M=79, Y=65, K=93 using the Isocoated v2 (ECI) profile, but converts to C=91, M=79, Y=62, K=97, when using the Coated FORGA 39 profile.

Which one is correct? Both, of course. This depends on the intended process and the requirements of your printing company.

(You say, you are working „for a client“. If so, I’d really recommend, to have a deep, deep look into to way, colour management is working. Customers deserve it and expect, that their designers understand their job. :))

 

[Kander]

thank you so much for taking the time to explain! 

It totally makes sense now. 

I totally agree about customers deserving and expecting their designers to understand their job which was why it was a concern for me that I didn't seem to be getting the right answers during my research and I'm so grateful for your response. I usually work with RGB and it's my first time designing a print product for a client. I keep seeing designers report that they first design in CMYK and then convert in RGB.. yet at the same time I see soooo many different CMYK profiles!   

[mac_heibu]

The preferred Color profiles vary from country to country. In Germany the most used profile is ISO Coated v2 300 (ECI). The 300 stands for a maximal tint coverage of 300 %.

[Kander]

Noted! I'll be sure to check with the client's printers on their preferred profile. Thanks again & have a great day!