The fabrication, in situ spectroelectrochemistry and colour measurement of hybrid electrochromic devices (ECDs) based on a surface-confined metal hexacyanometallate - Prussian blue (PB, containing the iron(III) hexacyanoferrate(II) chromophore) - and aqueous solution-phase methyl viologen (N,N'-dimethyl-4,4'-bipyridylium) are described. In the ECDs, the initial ('off') bleached state is set with PB in its reduced form and the methyl viologen as the di-cation. Switching to the coloured state ('on'), forms the mixed-valence iron(III) hexacyanoferrate(II) chromophore upon oxidation of iron(II) hexacyanoferrate(II), with simultaneous reduction of the methyl viologen di-cation to form a mixture of the radical cation monomer/dimer. Using the Commission Internationale de l'Eclairage (CIE) system of colorimetry, the colour stimulus of such ECDs and the changes that take place on reversibly switching between the colourless and coloured states have been calculated from in situ visible region spectra recorded under electrochemical control. The concentration of the solution-phase methyl viologen and its diffusion to the cathode controlled both the proportion of surface-confined (reduced) PB that is switched to the blue form and the overall ECD changes. For the ECDs' 'on' states, the CIELAB 1976 colour space coordinates for a D55 illuminant were L*=60, a*=3 and b*=-46, and L*=49, a*=9 and b*=-59, respectively for 5 and 10 mM methyl viologen solution concentrations. The low a* and high (negative) b* chromaticity coordinates quantified the overall ECD colour stimulus of the 'on' state as being deep blue, with a broad absorption across the visible spectral region. CIELAB 1976 colour space coordinates showed that the ECDs were fully transparent and nearly colourless in the 'off' states, with L*=100, a*=1 and b*=1. The changes in the transparency were 83.0% (5 mM methyl viologen) and 93.1% (10 mM methyl viologen) between the 'off' (bleached) and 'on' (coloured) states of the ECDs. (C) 2011 Elsevier B.V. All rights reserved.