RGB and CMYK: Why both?
Any of you who have at least little experience with graphic design software, photography or professional printing, must have -almost undoubtedly- have gone through this: 'the struggle' of RGB and CMYK colour spaces.
Now, there are many questions regarding matters of 'which should I choose', 'what is what' and converting problems, but that's not really what I'm after here.
As a graphic designer with only a couple of years of experience, I still have this struggle from time, and find the whole CMYK/RGB choosing/converting/saving situation quite confusing, to say the least. I do know that RGB is meant for digital end purposes, and CMYK is suitable for printing.
I have always been wondering about the neccesity of this whole thing. So, my question is: why are there these two colour profiles, instead of just having one?
Wouldn't it be a lot easier if printers or software could convert any RGB file to a CMYK printing document instead? I'm pretty sure it does already exist, by the way, because printing an RGB file sometimes simply prints the right colours for me. Alas, not always (doesn't seem to be related to the printer of software I print from).
I have gone trough some RGB vs. CMYK-related questions and found this, a.o., (on this page), an answer of user DKuntz2 (thanks DKuntz2):
RGB is a light-based theory. All colors begin with black "darkness", to which different color "lights" are added to produce visible colors. RGB "maxes" at white, which is the equivalent of having all "lights" on at full brightness (red, green, blue).
CMYK is a color-based theory. All colors start with white "paper", to which different color "inks" are added to produce output colors. CMYK "maxes" at black, at which point all "inks" are applied at 100% (cyan, magenta, yellow, black).
Not sure if this is entirely right, though, because some people disagreed with DKuntz2:
I disagree. With RGB you start with black - the absence of light; with CMYK you start with white paper.
Although this does clarify the differences fairly well, I still don't see the point of the two colour profiles(/spaces)
Side question: What happens when you work in an RGB document in photoshop, then press
ctrl+y? Photoshop says the document is
RGB/8/CMYK, but it can't be both. Right?
Big thanks for the answers, the (current) three below are very informative and interesting. I understand the whole thing a bit better, although it's all very complicated and in-depth to me. Especially when I read about terms like colour models, colour profiles, colour spaces, colour spectra and gamut all one after another.
As much as I would like to accept an answer as the desired one, I feel that my question in the core isn't fully answered yet. What I would really like to emphasize is what I asked about earlier:
Wouldn't it be a lot easier if printers or software could convert any RGB file to a CMYK printing document instead?. As Alan Gilbertson explained, the reason is - if I understand correctly - basically a 'lost-in-translation' thing that makes the conversion not 100% accurate, and the colours to possibly be a bit distorted.
However, I still don't see how the solution cannot be an automated conversion done by a printer. Should be possible with modern technology; if you can convert RGB to CMYK in Photoshop, why can't a printer do the exact same for you? As was also said in the answers, the slightest differences in colours and colour spaces, can not be seen by the human eye. And that's what it all comes down to: what we want to see. Be it on a screen or on a sheet of paper.
The fact that this question has three very long answers, or even moreso the fact that there are so many questions about these two colour models (I now know the right term, yay) kind of proves that this is way more complicated than it should be.
RGB is an additive spectrum... you ADD colors to get white. Dkuntz is correct stating that RGB is light-based. It is. It uses the visible light spectrum to display colors.
CMYK is a subtractive spectrum... you REMOVE color to get white. DKuntz's use of the term "color-based theory" is really nonsensical. Since RGB is also a color spectrum. A more appropriate term would be ink-based system.
Additive and subtractive are the preferred terms in my experience.
RGB can produce a larger range ("gamut") of colours than CMYK. The difference in this range is critical when designing. Often very vibrant RGB colors can not be reproduced in CMYK, therefore those colors (called "out of gamut") must be altered to fall within the CMKY gamut, or range of possible colors.
With the advancement of software and color management in digital environments (desktop PCs, imagesetters, platemakers, presses, etc.) the need to separate the two implicitly has diminished to a degree because software has gotten smart enough to make wise decisions when converting between the two. More specifically, when converting from RGB to CMYK.
However, digital color management has not always been present. It has only become reliable and usable in the past 5-7 years. And really only solid in the last 3-5 years. Before that results could be wildly inaccurate unless you had someone who had an eagle eye for color and could color correct anything after converting from RGB to CMYK. Or in some instances, color management was not anywhere present in a press environment. This is why a designer, working digitally, had to manually convert color spaces before sending anything off to press. That way if there were any color alterations, due to out of gamut colors, they could be addressed before anything hit the presses and started running up the bills.
Most Adobe software, as well as many other packages, now will convert RGB to CMYK upon output when needed. So, to a degree, things may seem to have become invisible. For example, if you export a PDFX1-a file from Adobe Indesign, and the Indesign file contains RGB images, Indesign will automatically convert the RGB to CMYK upon export. This conversion is based upon color profiles embedded in images/documents as well as color profile settings for the application. Incorrect color profiles will yield unexpected results on paper.
I still find it a very valuable practice to manually convert from RGB to CMYK as a final step in all image editing, if the image is destined for press. It is much easier to convert to CMYK, then spend 15-20 minutes tweaking any colors as needed than it is to reprint a large run of physical pieces (and its cheaper).
There are, of course, mountains of books and web sites you can read to learn more about the differences between the two color spectrums.
The side question....
Command-y in Photoshop is "Proof Colors" this means you are still working in RGB, but Photoshop is simply displaying the image as it will appear in CMYK (based on your working profiles). Essentially, you change what you see without altering the image itself.