Invented in 1798 by Alois Senefelder, offset lithography works on a very simple principle: ink and water don't mix. Six-color offset lithography or printing works by first transferring an image photographically to thin metal, paper, or plastic or printing plates. Each of 6 colors has a separate plate. Even though you see many, many colors in the finished product, only 6 colors are used.

In offset lithography the image on the printing plate is not recessed or raised. Rollers apply oil-based ink and water to the plates. Since oil and water don't mix, the oil-based ink won't adhere to the non-image areas. Only the inked image portion is then transferred to a rubber blanket or cylinder or drum that then transfers the image onto the paper as it passes between it and another cylinder beneath the paper. The term “offset” refers to the fact that the image isn't printed directly to the paper from the plates, but is offset or transferred to another surface that then makes contact with the paper.

The paper is left slightly wet by all of the ink and water being applied. Smudging is avoided by having the paper pass through an oven. Immediately after leaving the oven, the paper is run through a short series of large metal rollers that have refrigerated water flowing through them. These chill rollers cool the paper down instantly and set the ink into the paper.