I’m wondering exactly what the physics are underlying printing. I understand the basics of subtractive color-mixing, but this doesn’t explain to me why certain colors are difficult or impossible to create with four-color printing, why good blacks are actually not done entirely with black ink, and why there are printing techniques that use more than four inks. What’s the difference between six-color printing and four-color printing? Why are some colors hard to reproduce? Are there other commonly-used color printing techniques that don’t use normal subtractive color-mixing?
Six-color printing is usually four-color plus a few special “spot colors” --particular shades that can only be reached by premixed colors, or glosses or metallics. The pigments used for CMYK make a good range of colors, but can’t get every color the human eye can perceive. That’s why we have spot colors, if you really need EXACTLY that color. But it’s expensive to add extra inks, so you don’t unless you have to.
Black, even if printed all black, just doesn’t have enough ink to look “true” black. It’d look blobby and fat in text areas if you run the black ink more than once over all the black areas. So you overprint it in non-text spots with some cyan or magenta, and that way you don’t need another ink run, just the usual CMYK.
The other “six-color printing” is called hexachrome which is also called “six-color process”. As Gaudere said, the range of colors (also called gamut) achieveable by CMYK is not that wide. Many things (such as your RGB monitor), have a wider gamut (which is why, if you’ve worked with photoshop, you’ll now and then see a “gamut alert”. Meaning, you can see this color on the screen, but a printer cannot reproduce it accurately).
Hexachrome was developed to improve the gamut of printing. While it has been around for a while, it is still pretty specialized. I’d WAG that 99% of all printing you see is CMYK or less. Some artbooks might be Hexachrome, however.
As to a couple of your other questions.
Another reason why blacks are hard to achieve is that printing ink is not perfectly opaque. It is a bit transluscent which is why the paper you print on is important. It will change the character of the images you print. As black is the absorbtion of all light, you’re not going to get a perfect black.
I don’t think any printing method is possible without subtractive color system. The physics just don’t hold up. See the link astro put up. You could do a print job with all spot colors, but I don’t think that’s excactly what you were asking.
[QUOTE=Gaudere]
[SNIP!]Six-color printing is usually four-color plus a few special “spot colors” --particular shades that can only be reached by premixed colors, or glosses or metallics. The pigments used for CMYK make a good range of colors, but can’t get every color the human eye can perceive. That’s why we have spot colors, if you really need EXACTLY that color. But it’s expensive to add extra inks, so you don’t unless you have to. [/SNIP!]
More to the point, trying to produce a constant shade through several signatures (groups of 16 or 32 pages all printed on the same sheet of paper) of a book or journal is difficult if not impossible if you’re also matching color art on the pages. The easiest (but not the cheapest) way to avoid this is to print the constant shade as a spot color using the Pantone Matching System, where shades of ink are built up from formulations provided by the Pantone company.
This site has a neat applet that shows the limitations of using three colors, even apart from the practical problems like getting a good black. Given three colors, you can only make the colors within the triangle in the Chromaticity Diagram. The site doesn’t let you work with four or more colors, but you can imagine how you could cover more of the colored area if you had four or five points to work with.
Note that that isn’t the “real” Chromaticity Diagram, but just an approximation since the colors are only what can be produced by your monitor.
Fascinating! Thank you all for answering.
Hexachrome shows up in the weirdest places.
It’s becoming more and more used in packaging … and not necessarily high-end packaging. You know the Great Value brand of grocery products at Wal-Mart? They’ve got GV cereal, and coffee, and potato chips, and soy milk, etc.
Well, the Great Value cereal boxes are printed in Hexachrome. When you open the box, on the inner top flap you can see the color test strip with little GCMYKO squares all lined up neatly in a row.