Why do professional print houses use gamut-limiting CMYK?

Question

I recently had some business cards printed, and was warned to use the CMYK colour space if providing TIF files. When I tried saving from Paint Shop Pro to a CMYK, of course the colours came out very much duller, so I provided a different file type that preserved my colours using RGB. To my horror, the cards came back nevertheless converted to the CMYK dull shades.

A few months previously I had been in a hurry and had printed my own one-sided quasi business cards using glossy photo paper and a cheap home printer. This reproduced my desired colours perfectly.

Since the whole point of using professional printers is quality (though I admit bulk as well), why do they not use printers that can cope with the RGB colour range? FYI: the main colour in question, which I chose very carefully, is a slightly yellow-tinged lawn green, which I understand doesn't feature in the CMYK gamut. So why use that gamut, and is there anything I can do to overcome this issue and get cards printed in the colour I have chosen?

Answer 1

As Janus suggests, it sounds like the question is actually:

why do printers use CMYK?

First of all, let's clarify RGB vs. CMYK. RGB is using the additive color model...meaning the colors are made from projected light. You add red, green and blue together to get pure white light. CMYK is using the subtractive (also called reflective) color model. It produced color my 'removing' light that is being reflected. Adding C, M, Y together will give you black (* at least in theory, in reality, it gives you a muddy brown, but that's a different topic...)

RGB and CMYK can both produce colors the other can't:

Let's look at the reasons why CMYK is used widely:

1. History

The use of RGB color is relatively new. We've had printed materials centuries before we had color screens. CMYK came about in 1906, about 60 years before a strong need for RGB color systems to take hold with color television and computer screens.

2. Standardization

Printing is big business (albeit perhaps a bit bigger a few decades ago) and having a universal standard way to print colors is a necessity. A company may need printing done in a dozen places around the globe and they're not going to want to create separate files for each print to accommodate a custom color system. As such, standardization becomes important. CMYK is one standard.

3. Lack of demand/need

It'd be nice if what we saw on screen matched what we put on paper exactly, but that's a bit of a fool's errand as there is simply a nearly infinite range of screens and papers out there in the world making it simply impractical. As such, those in the design and printing industries have managed to cope just fine using things like ink swatch books, proofs, and press checks.

4. Alternatives

And we do have alternatives to CMYK. The most common would be spot colors (Pantone being the most popular classification system for them). With spot colors, you can print well outside both the existing RGB and CMYK spectrums and create all sorts of colors that can't be produced with either such as day-glo, metallic, etc. There' also CMYK+ systems that use CMYK but then add several other colors. These tend to be most common with high-end personal printers for printing limited edition fine art prints.

With the advent of digital printing, we may see more and more CMYK+ options out there for limited run printings.

Answer 2

Large format offset printers print in CMYK at the fundamental level in what is called "4 color process printing".

That is because the drums that are used to transfer the ink to the paper do it one color value at a time. Ink is usually printed in the order of the abbreviation Cyan, Magenta, Yellow, and Key ( Black ).

In the RGB color model all three colors added together create white, while in real life inks mixed together do not. They create black.

It's pretty much impossible to print in RGB since the color values emit white as a value that can only be generated on a computer screen / TV etc.

Your desktop printer may print in Spot color (which is still CMYK) which is able to expand the possible colors printed.

Why doesn't your professional printer? Because of cost to value. 4 color process is the industry standard especially for business cards or fliers, etc. You may be able to find a good wholesaler who uses spot coloring for business cards but I doubt.

Your best bet in the future is to buy a CMYK color chart (Pantone sells one) and choose your best match before sending to the printers.

Answer 3

Your specific printing issue is that subtractive printing means that adding color (ink) makes the result overall darker. Mixing the proportion of C and Y to get the shade of yellow-green you want will have a maximum brightness that’s below that of the white paper. If you use less ink overall you get more white from the paper which is all color, thus it becomes less saturated, not brighter in the same shade.

The brightness is relative to the rest of the picture. So make everything dark to match... that might be what the print shop did.

Maybe you would prefer less saturation to being darker... printing at home, you decide through iterative feedback what you want.

I would think the rgb to cmyk done at the print point would use the profile to match the equipment, and possibly do better than a generic conversion. But you can't guide the process! So use the proofing feature in Photoshop (or whatever) to visualize how it will come out, and choose the compromises yourself to make the overall appearance look best.

Finally, a desktop ink-jet may be more photo quality than a commercial printer. Recently I saw how the regular color output at FedEx (the photo print kiosk wasn't working) was not as good as my printout from home.

My info is not up to date, but I think the toner technology used by those print shop machines don't have as bright of primitives as the liquid dye or pigment in an ink jet. If the plain yellow is darker, so is everything that mixes it.

Getting the profile to use will show you (in proofing) what it can do.

If you want bright images, and production-run speeds, you need a dye-sublimation printer or a photographic paper print.

Answer 4

You can overcome many of the restrictions if your willing to pay more. Because the print word is inverse* of what the monitor does it means that in order for the monitor sRGB space fit to a print space it would need quite impossible inks. On the otherhand there are quite many colors that it can print that your monitor can not show.

One can overcome this by having more inks. By printing with 6–7 colors you could have a much bigger range of course its nolonger cheap. Many inkjets however do this. Inkjets in general are quite superior to many presses, but also many times more expensive. You could also use pantone colors this may also be more expensive but now yo can choose really interesting colors.

Sending a RGB file is also many times a mistake. With cmyk files you can adjust to fix the dullness. If you send them RGB they will just do the conversion automatically and you have no sayso of how it happens.

* monitors send light, or other words add it. Paper reflects it or subtracts light.