Lithography is an ingenious printing process which involves a flat plate, usually made of limestone, where the surface has been treated with hydrophobic and hydrophilic chemicals so that when ink is applied to the plate it doesn't stick to the hydrophobic regions and moves to the hydrophilic regions. The plate is then pressed against paper to leave a print of an image.
The above method would of course produce a monochrome image, though during the 1890s color lithography was becoming increasingly popular with French artists using multiple plates, one of each color, and then printing one color at a time onto the paper. The main problem with this method was trying to keep each of the plates perfectly aligned. If they weren't, the colors wouldn't be in register.
Before then it was common for the Lithographs to be hand tinted like the ones below.
Printing of erotic images was at a peak in France from the 1800s to the early 1900s when they were very popular with tourists and sold as postcards, though very few of these "postcards" could actually be sent through the postal service due to stricter regulations of pornography. In fact, many of the images produced were illegal at the time and thus the majority of erotic lithographs are by unknown artist wishing to remain anonymous.
Below is a series of erotic lithographs by an unknown artist, though judging by the style they are most likely the work of Achille Deveria.