[1] The term ‘chromic’ is defined for materials that can change colour reversibly with the presence of an external factor.
The colour change of halochromic substances occur when the chemical binds to existing hydrogen and hydroxide ions in solution.
[1] Such bonds result in changes in the conjugated systems of the molecule, or the range of electron flow.
This alters the wavelength of light absorbed, which in turn results in a visible change of colour.
The various shades result from different concentrations of halochromic molecules with different conjugated systems.