On July 31, 1804, English artist and printer George Baxter was born. Baxter is credited with the invention of commercially viable colour printing. Though colour printing had been developed in China centuries before, it was not commercially viable. Baxter used wood and metal colour blocks in conjunction with steel key plates and using oil inks.
George Baxter was born in 1804 in Lewes, Sussex, the second son of John Baxter (1781–1858), printer and publisher at Lewes, who issued the popular illustrated “Baxter” Bible. At 20 Baxter was illustrating books printed by his father; at 23 Baxter moved to London to be apprenticed to Samuel Williams, a wood engraver. In 1827 Baxter set up business on his own and married Mary Harrild, daughter of Robert Harrild, a printing engineer and a friend of Baxter’s father. Baxter now began to experiment with his own methods of colour printing – his first known colour print, Butterflies, was published in 1829. Baxter’s experiments first bore commercial fruit in 1834, with two small vignettes published in Mudie’s “British Birds”.
Little of Baxter is then known until 1834 when he began to produce a number of colour prints which were used as frontispieces in books published by Robert Mudie. By 1835 Baxter had developed his method of colour printing sufficiently to apply for a patent, which was granted as Patent No. 6916 – Improvements in Producing Coloured Steel Plate, Copper Plate and other Impressions, which outlined the combined intaglio and relief process he would continue to use for the next thirty years. His process meant that colour printing could be achieved relatively cheaply and for the first time made colour available to all. Baxter’s original patent ran for 14 years; after the renewal of his patent in 1849 for another five years he began to sell licenses for the use of his printing process to other printing firms.
My invention consists in colouring such impressions of steel and copper plate engravings and lithographic and zincographic printing by means of block printing in place of colouring such impressions by hand as heretofore practised, and which is an expensive process; and by such a process producing coloured impressions of a high degree of perfection and far superior in appearance to those which are coloured by hand and such prints as are obtained by means of block printing in various colours uncombined with copper and steel lithographic or zincographic impressions. (from Baxters Patent No. 6916 )
Although he sold his work to several highborn clients and was even summoned to visit Queen Victoria, he had little business sense and did not profit greatly from his invention, partly because he encountered competition from cheap coloured lithographs. Baxter used carefully etched plates, a handpress, a system of register points, and the finest colours, oils, and paper. It seems likely that his perfectionism prevented him from completing many of his commissioned works on time.
Baxter retired from active business in 1860, but the sale of his prints and materials was a failure and much of the material was bought in by the auctioneer. He began to republish his prints late in 1864 or early in 1865. Baxter was declared bankrupt in 1865 and died in 1867, after an accident involving a horse omnibus.
Despite experiments in colour printing already started in the 16th century, most prints were still either monochrome or hand coloured in the late 1820s. This was very labour intensive and therefore very expensive. Baxter, however, brought together various methods of printing which enabled prints to be made in colour which were both effective and relatively cheap. Baxter’s process for producing colour prints combined relief and intaglio printing methods. A ‘key’ plate was prepared, usually made of steel and using any combination of engraving, stipple, etching and aquatint. Baxter also appears to have used mezzotint and lithography to create his key plate on occasion.
The key plate provided the main lines of the image and much of the tone, light and shade. It was usually printed in a neutral tone, such as light grey or terracotta. Often Baxter used more than one colour to ink the key plate – for example, to gradate the image from blue in the sky, to buff in the middle distance and to a darker colour in the foreground; i.e. inking the plate à la poupée. Sometimes up to 24 separate colours were used, although ten could be considered an average number. Baxter achieved his precise registration by fixing the print over a number of spikes, over which the blocks would also fit. Some of the prints required only 8 different blocks but some involved as many as 20 different colours, each being superimposed on the other. 
At yovisto, you can learn more about the history of printing in an educational video from the 1960s.
References and Further Reading:
-  George Baxter, British engraver and printer, at Britannica Online
-  George Baxter Biography, at Victoria University Library
-  George Baxter at The New Baxter Society
-  H. George Clarke: Baxter colour prints; their history and methods of production, and other interesting matter relating to operators of processes akin to his methods, together with Baxter & LeBlond auction records for 1917-18.