Four Color Process
A process color is printed using a combination of four standard process inks: cyan, magenta, yellow, and black (CMYK). Use process colors when a job requires so many colors that using individual spot inks would be expensive or impractical, such as when printing color photographs.

The four-color process is actually a pattern of tiny dots, which are round, elliptical, or square. By there density, these dots produce disparity of both value and color saturation for the final image. Screens that generate these dots vary according to the desired resolution of the printed images and the type of paper onto which it is printed. Halftones are a pattern of dots used for reproduction of a continuous tone image. These dots gradate in size to suggest light and dark areas of the image by optical mixtures. Dots in dark areas are larger to cover more white paper. Dots are smaller in light value areas so that the white paper optically mixes with a small amount of color to produce tints. Gradation of the size and overlapping of dots produces the illusion of a smooth value, hue, or tonal gradation. The screen pattern of dots or dpi (dots per inch) can be coarser or finer depending on the desired resolution of each printing job. Duotones are two-color halftones achieving more depth and contrast in an image. Two closely related colors or black and an accent color could be used. The image is captured twice emphasizing the highlights in one and shadows in the other. The combination of CMYK dots in various sizes and overprinting colors forms optical mixtures that give the illusion of a wide array of colors. The first step in this process is a color separation. Each separate color of CMYK is screened to create a dot pattern. The dots are graded in lpi (lines per inch) making up the screen frequency. These can be roughly sorted in quality as follows:

    Newspapers: 85 lpi
    Magazines: 133-150 lpi
    High Quality art reproductions: 300 lpi

The four-color process is printed in layers in this order: first yellow, then magenta, then cyan, and then black.

Registration is important with four-color printing process. Registration must be precise to prevent a blurry or misaligned image. Trapping can be found in software and is used to overlap the colors to compensate for misregistration.

Keep the following guidelines in mind when specifying a process color:

• For best results in a printed document, specify process colors using CMYK values printed in process-color reference charts, such as those available from a commercial printer.

• The final color values of a process color are its values in CMYK, so if you specify a process color using RGB, those color values will be converted to CMYK when you print color separations. These conversions will work differently if you turn on color management; they'll be affected by the profiles you've specified.

• Don't specify a process color based on how it looks on your monitor unless you have set up a color management system properly and you understand its limitations for previewing color.

• Avoid using process colors in documents intended for online viewing only, because CMYK has a smaller color gamut than a typical monitor.

Offset-litho Printing
With offset-litho printing, paper is loaded onto a platform at the back end of the press above which is a series of suckers.

These suckers lift one sheet at a time and enable the "grippers" (grabbing pincers mounted on a chain-link drive) to grab the sheet along one edge (called the gripper edge) and pull it into the first "ink unit," where it is printed with the first color.

As this sheet leaves, it is grabbed again and pulled forward and into the next ink unit, and so on. Presses vary as to how many ink units they have. Presses typically used for color work have at least four units (a cyan unit, a magenta unit, a yellow unit, and a black unit) and maybe a fifth for a varnish or spot color.

At the top of the ink unit is a trough containing ink. Sitting in the trough is a roller which turns slightly every few seconds. Behind it is another roller which jumps forwards to touch it, picking up a long bead of ink, and then jumps backwards to spin against the other rollers in what is called the inking system.

This is a complex series of rollers designed to bring the ink down onto the plate in a controlled way. Usually only three or four rollers out of the whole array actually touch the plate surface and transfer the ink to it.

Most people think that the image is transferred to the paper by the plate, not so. The plate is a thin sheet of aluminum (sometimes zinc or another thin material) and has a hard surface. It would quickly wear itself out if it had to come into contact continuously with a comparatively abrasive material like paper for very long. Instead, the image is transferred ("offset") to the blanket cylinder, a roller covered with a canvas-backed sheet of rubber. Hence the term "offset printing."

The blanket, being rubber, can press down hard on the paper with no problem. Simultaneously, the impression cylinder is pressing up from underneath. The resulting pressure keeps the paper from slipping as it moves between the two cylinders, thus creating a sharp image.

After the last unit of the press, the paper hits a stop bar just as the grippers let go of it and settles onto and ever-increasing pile of printed sheets. Typically, sheet-fed presses are run at around 3,000 sheets an hour.

(see image below)

(adapted from Getting it Right in Print, Mark Gatter, Harry N. Abrams, New York. 2006)