I wonder when colourful advertisements became commonplace in print (such as magazines or occasionally newspapers). I can imagine that colourful printing was far more expensive - so at what point did it became more affordable? What technology, if any, was responsible for this?
Wikipedia claims that, while color printing was known even in the earliest printed works, it was various Chromoxylography processes developed in the 1800's that first made color printing practical enough to be commonplace.
In the 19th century a number of different methods of color printing, using woodcut (technically Chromoxylography) and other methods, were developed in Europe, which for the first time achieved widespread commercial success, so that by the later decades the average home might contain many examples, both hanging as prints and as book illustrations
The key development was not actually in printing technology, it was in photography. Being able to print in color is useless unless you can generate color images in the first place. The critical invention was Kodachrome which became widely available in 1936. At roughly the same time advances in lithographic presses allowed the images generated from Kodachrome to be printed in color relatively cheaply. If you go back and look at old magazines you can see that they switched to color right around 1936. Time Magazine first started to print in color in 1938.
People used paints from the very ancient time. When first books appeared, people quickly accustomed to paint the pages and make colorful images. In the Middle Ages the majority of books published had colorful images in them.
The state symbols, coats of arms, military banners and religious icons were also colorful.
With the invention of printing press the number of colorful images in the books reduced because this technology initially did not allow to make colored images and painting them by hand would increase the cost. As time went on and technology progressed the images gradually returned to published books.