Re: What is the correct name for inks that change/revert colour.

Julian,

The class of inks that you are referring to are called "THERMOCHROMIC".  
These inks consist of dyes (sometimes encapsulated) whose molecular 
structure changes (usually from a colourless to a coloured form) with 
temperature.  Some of these dyes are reversible and are often used on 
toys; Others do not necessarily revert back to their original colour after 
a certain temperature is reached; these dyes are used in ink jet printers 
or copiers. Typically, these ink jet dyes are encapsulated; once the 
printer or copier heats them to a certain temperature and the colour 
develops, the other ingredients in the ink will preserve the dye in the 
coloured form...  For more information concerning these types of dyes, 
search the world wide web for "thermochromic inks (or dyes)".