CMYK vs. RGB

[mykee]

Hello,

I don't understand color conversion in AP. I have a color: #800000 what in RGB is 128,0,0 and in CMYK is 0,100,100,50. But in AP when I use CMYK project with Coated Fogra39, then I get different values with CMYK: 29,100,100,40

Ok, I set CMYK to standard value (0,100,100,50), but I get RGB 134,16,3 color value...

I don't understand why can I get different values on convert? This linked to ICC profile? Or why calculated different values? 

[mac_heibu]

There is a basic error!

First, you can‘t reproduce RGB colours exactly in CMYK, because the CMYK color spaces are much smaller as the different RGB colour spaces.

Second, and more basically, you can‘t simply convert CMYK values mathematically to RGB values and vice versa. Why? Because for printing quality reasons the color „K“ is added, which doesn‘t exist in the „physical“ colour system. To say it in simple words: The CMYK colour system is a mathematical „incorrect“ system, because a colour is added, which normally doesn’t belong to the system. A conversion like meter to inch would only be possible, if CMYK wouldn’t be CMYK, but CMY. CMY (additive model) and RGB (subtractive model) can be converted mathematically into each other.

Not only, but mainly because of the „K“,  conversions between CMYK and RGB need color profiles, which provide the needed conversion tables, which are set up to match the specific needs of a specific printing process with specific prining colours. Different color profiles necessarily leed to different colour values. So you will get different values if you convert usind a SWOP or ISO Coated.