How Printers Work

Creating Color

Like computer monitors or television screens, printers produce colors by tightly positioning the key primary colors in a process that is called dithering.
Monitors and printers, however, differ in how this is accomplished because monitors are light sources, whereas printing output is light reflecting. Monitors mix the light from phosphors through the primary additive colors: red, green and blue (RGB), while printers use inks from the primary subtractive colors: cyan, magenta and yellow (CYM). In both cases the basic primary colors are dithered to form the entire spectrum. Dithering breaks a color pixel into an array of dots so that each dot is either made up of one of the basic colors or intentionally left blank.
Reproducing color viewed on a monitor to exactly match printer output is known as color matching. Colors vary from monitor to monitor and the colors on the printed page don't always correspond exactly with what is displayed on screen. The color generated on the printed page is dependent on the color system used and the particular printer model; not by the colors shown on the monitor.
Modern inkjets are able to print in color and black, but the way they switch between the two modes varies according to their capabilities. Printers enabled for four-color printing - cyan, yellow, magenta, and black (CMYK) - can switch between black and color images all on the same page with no problem. Printers equipped with only three colors cannot.
Many of the cheaper inkjet models have room for only one cartridge. You can set them up with a black ink cartridge for monochrome printing, or a three-color cartridge (CMY) for color printing, but they can't do both at the same time. This makes a big difference to the operation of the printer. Each time you want to change from black and white to color, you must physically swap the cartridges. When you use black on a color page, it will be made up from the three colors, which tends to result in an unsatisfactory dark green or gray color usually referred to as composite black.