How spot colour is done for letterpress
Colour printing can be letterpress printed like offset printing using the 4 colour process. This involves printing cyan, magenta, yellow and black to simulate a wide range of colours. Using letterpress techniques to do this is extremely rare, very expensive, time consuming and, limits the paper choices and techniques available for printing letterpress. Colours used in letterpress printing are created without screens or dots, such as those found in the PANTONE MATCHING SYSTEM®. From a palette of 18 basic colors, each of the spot colours is mixed according to its own unique ink mixing formula.
A separate piece of artwork is created for each colour to be letterpress printed. That artwork is transferred to film and from that a plate is made for each. The plates are mounted to the single colour press and printed in precise alignment on the paper.
The illustration below shows the process graphically.