Stanley Gibbons Colour Key.

Here is a nice example. Hong Kong issued this souvenir sheet. Next to the $5 stamp are the individual prints from the four plates used to print the stamps. Each plate printed a different ink. Note the third plate with the background and Chinese characters in the same colour. Note also the fourth plate printing a single colour on the whole printed area other than the territory's name and value that were reversed out of the printing. So, the portrait is in the same colour as the general printing.



And yes, there are four. The second is for a fluorescent ink printing of the territory's name.