When changing the CMYK to RGB colours, why is the colour different?
OK, first, everything that was said I this thread is true! Still I can remember when I was trying to understand, 25 years ago, what it was all about I missed the same information that is missing here:
WHY is CMYK a different colour gamut!? And what happens to the colours where?!
The gamut: Before ICC we were always talking about dot gain. You make a dot with a fountain pen on paper and it turns out bigger than the tip of the pen. In principle the same happens with the dots on the printing plate. So to get 50% paper covered you may only need 35% on the printing plate.
Some screen points are too small to even show and to get complete coverage you need less than 100%. That is the first reason that you have a smaller colour spectrum.
Second: Colored paper reflects less light than things in nature do and also less than your screen radiates.
Third problem: Theoretically you could mix all colours out of CMY, but in real life it is not like that. Therefore we ad black.
Fourth problem: There is not only one CMYK gamut. It all depends on the printing machine, the inks used, and the paper. In some processes more of the CMY colours are left out and substituted with black, than in others.
That is a lot of information to take into consideration. This information is gathered in the ICC profile.
So you take your RGB photo, change it into a certain CMYK for a certain printing process - it reduces the colours so that the printed product is as close to your RGB photo as possible.
You also send it to the proof printer. Based on the information in the ICC profile and it's own ICC profile the proofer will try to simulate what we can expect from the production print.
You don't see any change on your monitor, because it does the same: Show it based on its own RGB which probably is different from the photo.
BUT: You reduced the colours by converting to CMYK. In fact, with some CMYK profiles you get pretty close when you convert back, but it would be a matter of luck What is gone, is gone
Capisce? Now to get a feeling of what colour management can do for you, and what happens to your colours where, try a few things out. With the software's you have. Import images with or without conversion,same when you print them or simply write PDF.
This is how I figured out everything that nobody could really explain.
Short and simple rule I tell people : CMYK is for printing on the paper (your inkjet has four colors bottles, Cyan, Magenta, Yellow and Black), RGB is for shining on the screen. You are only ever going to see approximations.