To provide a functional spectral filter through which, it is possible for an observer wearing the functional spectral filter to change his or her color sensitivity (ease of color distinction), and which is for designing a color scheme easy to be distinguished for a color deficient observer by using the filter. A functional spectral filter including a multilayer is formed in accordance with a thin film design (an optimization method) determined by use of a color vision theory so as to make color sensitivity into a desired pattern. With respect to combinations that two colors among a plurality of specified colors are combined, color differences in the respective combinations are made close to a color difference given in advance. Alternatively, with respect to specified combinations, the color differences are made small or the color differences are made large.