Lithographic color printing is a relatively new process. It was popularized with the beautiful European color posters of the late 19th and early 20th century by artists the likes of Jules Cheret and Toulouse-Lautrec. Unfortunately, original lithography (hand pulled, whereby the artist hand draws all designs and supervises every aspect of the printing process, as opposed to the photomechanical reproduction process of offset lithography) is a dying art.
The lithographic process is a planographic process. That is to say, the printing and non-printing area of the stone or plate are at the same level, unlike intaglio and other relief processes such as the etching and woodcut.
The lithographic process is based on the fact that water and oil don't mix. To begin, the artist draws the image on the lithographic stone. The mirror image must be drawn so that the final state appears as desired. Specially prepared limestone was once used, but now, due to weight and ease of handling considerations the drawings are more often made on metal plates, usually zinc. It is rare indeed to find a printer or artist using lithographic stones today.
If a litho is to have 17 different colors, each piece of paper in the edition must be run through the press 17 times, 18 colors, 18 times. Let's say the artist chooses to start with the red color - with a grease pen or crayon he draws that part of the image that will be red and any area that will be a combination of red and another color. A red, oily ink that adheres to the grease is applied with a roller and the stone is then sponged off with water to remove ink from any areas not drawn. If the edition size is to be 100 pieces, the printer may run several hundred sheets of paper through the press for this first color. There will be many pieces discarded with later colors as refinements are made until the image is exactly to the artist's liking.
When all the colors have been printed and the artist and printer are happy with the final result, the first piece off the press is traditionally labeled the Bon à Tirer. Literally translated from French, good to pull. The sheets of paper are put through the press without attention to order. That is to say, the first sheet for the red color, may have been the 88th or 188th for the blue color. The sheets are signed and numbered with the same disregard for order, so there is absolutely no difference between piece #1 and piece #100.