Chromolithography became the most successful of several methods of color printing developed by the 19th century.
Hand-coloring also remained important; elements of the official British Ordnance Survey maps were colored by hand by boys until 1875.
The initial technique involved the use of multiple lithographic stones, one for each color, and was still extremely expensive when done for the best quality results. Depending on the number of colors present, a chromolithograph could take months to produce, by very skilled workers.
To make an expensive reproduction print as what was once referred to as a "chromo", a lithographer, with a finished painting in front of him, gradually created and corrected the many stones using proofs to look as much as possible like the painting in front of him, sometimes using dozens of layers.
Colors may be added to the print by drawing the area to receive the color on a different stone, and printing the new color onto the paper. Each color in the image must be separately drawn onto a new stone or plate and applied to the paper one at a time. It was not unusual for twenty to twenty-five stones to be used on a single image.
Each sheet of paper will therefore pass through the printing press as many times as there are colors in the final print. In order that each color is placed in the right position in each print, each stone or plate must be precisely ‘registered on the paper.
Offset printing, more cheapier process, replaced chromolithography in the late 1930s.
