Online support



Black and white, grayscale, and color printing


Offset printing press reproduced as a simulated black and white halftone image.


Traditionally, printing presses used a single color of ink (black) to produce basic black-and-white text, but printing photographs and artworks was much more difficult because they really needed to be printed either with many colors or many shades of gray. That problem was solved when people discovered how to simulate shades of gray using what's called the halftone method. It's a simple way of converting photographs and drawings into images made from tiny black dots of differing sizes to give the impression they're made from many different shades of gray. In other words, it's a way of making a convincing gray-scale image using only black ink, and it relies on fooling your eyes through an optical illusion.


Photo: Halftones: Here's the same photo up above as a newspaper might print it using different sized areas of black ink. If you squint, or look from a distance, you can see that it looks like it's been printed with many different shades of gray, even though it's really using only one color of ink (black). In practice, newspapers use much smaller dots than this—we've exaggerated greatly so you can see how it works. Photo by Senior Airman Dilia DeGrego courtesy of US Air Force, with simulated halftone treatment by explainthatstuff.


To print in full color, you need to use at least four different inks: three primary ink colors and black. Most people know that you can produce light of any color by adding together different amounts of red, green, and blue light; that's how a television or LCD computer screen works. Colored inks work in a different way by subtracting color: they absorb some of the light that falls on them and reflect the rest into our eyes—so the color they appear is effectively subtracted from the original, incoming light. If you have an inkjet printer with replaceable cartridges, you'll know that you can print any color on white paper using the three colors cyan (a kind of turquoise blue), magenta (a reddish purple), and yellow. Theoretically, you can produce black with equal amounts of cyan, magenta, and yellow, but in practice you need a fourth ink as well to produce a deep convincing black. That's why full-color printing is often referred to as the four-color process, sometimes as CMYK printing (Cyan, Magenta, Yellow, and K meaning "key," a printer's word that usually means black), and sometimes (since each color has to be printed separately) as color-separation printing. Just like with black-and-white printing, the halftone process can also be used to create varying shades of color.


Bottles of black, cyan, magenta, and yellow ink used for four-color printing.


Photo: Color printing: With black, magenta, cyan, and yellow ink, you can print any color you like.


Why is color printing more expensive?


Printing in color costs much more than printing in black-and-white, for various reasons. First, and most obviously, there are four inks involved instead of just one and each is printed by its own printing plate, so the cost of making the printing plates alone is several times greater. Second, color printing presses need to be able to print the four inks on the page one after another, in perfect alignment, so they need to be considerably more sophisticated and precise. Third, it takes extra time and effort for the person operating the printer to check that the colors have been aligned and reproduced successfully, so there's more human effort involved. Finally, because color printing is often used for reproducing photographs, heavier, glossier, and more expensive paper is usually needed to do it justice.


Sometimes designers get around the cost of color printing by using different colored papers and inks. So, instead of printing black ink on white paper, they might print black ink on red paper or red ink on yellow paper. That achieves a colorful effect but keeps the cost down by still using only a single color of ink. Another option is to use spot-color printing, where a single, specially mixed color is applied to a black-and-white document—though that is labor intensive and can work out even more expensive than four-color printing. Another alternative is to use two- or three-color printing, in which pages are printed with black and one or two other colors. If you were using just cyan and magenta inks, for example, you could create a whole range of reds and blues and print quite colorful pages without the expense of four-color printing. Another option is to print some pages with the four-color process and other pages with only black-and-white. Books that contain photographs are often made this way, with the art pages printed through the four-color process on glossy paper that's bound inside text pages printed with the black-and-white process on ordinary paper.