A method for determining viewers' individual color matching functions, the method comprising: selecting only two metameric unstable image displays, wherein each one of the only two metameric unstable image displays is configured, when rendering a color, to cause different viewers to see or visually perceive different colors with the same rendered color, the two metameric unstable image displays having different spectral power distributions, the spectral power distribution of each of the only two image displays representing the power radiated by the image display at wavelengths of the visible spectrum, displaying two corresponding color patches on the only two image displays, at least one of the two corresponding color patches being adjusted by a viewer to match visually the two corresponding color patches to a common color as visually perceived by the viewer; subsequent to matching visually the two corresponding color patches to the common color, performing: identifying two sets of code values rendered on the two corresponding color patches on the two image displays; determining, based at least in part on the identified two sets of code values, two spectral power distributions of light displayed on the two image displays used to render the two corresponding color patches; generating, based at least in part on the determined two spectral power distributions, the viewer's color matching function (CMF); causing the viewer's CMF to be used in image rendering operations on a target image display. selecting only two metameric unstable image displays, wherein each one of the only two metameric unstable image displays is configured, when rendering a color, to cause different viewers to see or visually perceive different colors with the same rendered color, the two metameric unstable image displays having different spectral power distributions, the spectral power distribution of each of the only two image displays representing the power radiated by the image display at wavelengths of the visible spectrum, displaying two corresponding color patches on the only two image displays, at least one of the two corresponding color patches being adjusted by a viewer to match visually the two corresponding color patches to a common color as visually perceived by the viewer; subsequent to matching visually the two corresponding color patches to the common color, performing: identifying two sets of code values rendered on the two corresponding color patches on the two image displays; determining, based at least in part on the identified two sets of code values, two spectral power distributions of light displayed on the two image displays used to render the two corresponding color patches; generating, based at least in part on the determined two spectral power distributions, the viewer's color matching function (CMF); identifying two sets of code values rendered on the two corresponding color patches on the two image displays; determining, based at least in part on the identified two sets of code values, two spectral power distributions of light displayed on the two image displays used to render the two corresponding color patches; generating, based at least in part on the determined two spectral power distributions, the viewer's color matching function (CMF); causing the viewer's CMF to be used in image rendering operations on a target image display. <EOS>