Colour-Change Ink changes the colour of the writing as it is being used.
Harry Potter replaces the top onto a bottle of Colour-Change Ink when packing his things into his trunk before leaving the Dursleys’ house with Albus Dumbledore for his sixth year at Hogwarts School (HBP3).
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Notes
The Streeler is a dangerous magical beast resembling a giant snail that kaleidoscopically changes colour every hour (FB). It may be that the Streeler provides an ingredient for the colour change element of this ink.
Colour-Change Ink appears in a Diagon Alley shop seen in the first Harry Potter film (CS/f).
From the Web
Harry Potter Wiki: Colour-Change Ink
A UK company called ColourChange markets inks that respond to changing temperatures: Thermochromic ink
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Tags: change color change quills writing