A reference-color measurement step of obtaining a reference-color measurement value by measuring a spectroscopic-radiation luminance of a light being emitted from a reference-color portion in a measurement direction, or a tristimulus value thereof, using a light-source-color measuring instrument, without irradiating the reference-color portion with a light source for measurement, in a predetermined measurement environment, is disclosed. An objective-portion measurement step of obtaining an objective-portion measurement value by measuring a spectroscopic-radiation luminance of a light being emitted from a measurement-objective portion in the measurement direction, or a tristimulus value thereof, using the light-source-color measuring instrument, without irradiating the measurement-objective portion with a light source for measurement, in the measurement environment, is also disclosed.