Image processing device, image processing method, endoscope system, and endoscope observation method

An endoscope observation method comprising:

• acquiring a reference image by capturing, by means of an endoscope, an image of a reference subject which has an optical characteristic that is equivalent to at least a part of a living body;
• calculating, on a basis of the acquired reference image, image transformation parameters through congruence transformation that transforms a coordinate corresponding to a color of a target region included in the reference image defined in the color space into a cooridinate corresponding to an achromatic color in the color space;
• acquiring a color image constituted by at least two monochromatic image corresponding to different illumination having different center wavelengths by capturing an image of the living body by means of the endoscope;
• performing the congruence transformation of all colors of the acquired color image in the color space on a basis of the image transformation parameters acquired by calculation;
• and
• displaying an image that has been subjected to the congruence transformation.