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Thermochromic dyes have been used as chemical indicators and for recording paper

crystal violet lactone
spiropyrans
fulgides

Strictly, these dyes are not thermochromic but change colur in response to a change in pH

phenolphthalein and thymolphtalein are colourless in acidic to neutral pH but become pink and blue in akaline environments.
redox indicators

Electron acceptors - or more strictly proton donors

bisphenol A,
octyl p-hydrobenzoate
methyl p-hydroxybenzoate
1,2,3-triazoles
4-hydroxycoumarin

All weak acids. Weak acids give reversible colour change, strong acids irreversible chnage

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