The endoscope system, a method of operating the endoscope system processor unit and an endoscope system

First illumination light (BN) of a first wavelength range, in which light absorption coefficient of blood hemoglobin varies with oxygen saturation thereof, is projected into a test subject body, to capture a first image signal from the first illumination light as reflected from inside the test subject body. Then second illumination light (BB) of a second wavelength range different from the first wavelength range is projected into the test subject body, to capture a second image signal from the second illumination light as reflected from inside the test subject body. A subject image of the test subject is produced from the second image signal. Oxygen saturation levels of the test subject are calculated using the first and second image signals. According to the calculated oxygen saturation levels, color properties of the subject image are changed to produce an oxygen saturation image.