Shipton must have been smart enough to notice the different color inks.
Certain kinds of copying inks are greatly improved by the substitution of glycerin, in part or entirely, for the sugar or honey usually added.
It is certain, however, that the mere use of colored inks must soon have suggested the combination of two or more of them, and it is probable that examples of this will be discovered much earlier in date than those known at present.
Linseed oil is also the principal ingredient in printing and lithographic inks.
One inks the type-forme and keeps a sharp look-out for any inequality of inking, and sees generally that the work is being turned out in a workmanlike manner.