Methyl ketone has been designed as a switching unit for electrically addressable molecular colour switches. A newly proposed mechanism of “electro-acid/base” (radical ions)-induced intermolecular proton transfer for the colour switch is proven clearly by cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR) and in situ UV-Vis spectroscopy. A dramatic spectral absorption shift (about 291 nm) is observed during the switching, and blue, yellow and green colours are obtained by adjusting the substituents on the methyl ketone-bridged unit. The in situ “electro-acid/base” is far more convenient than the conventional chemical stimulus of acids or bases for the manipulation of the molecular switching properties. This new switching method and molecular structure manipulation will inspire and accelerate the further development of broad switching materials and applications in ultrathin flexible displays, etc.