Image processing device, method for operating the same, and endoscope system

First RGB image signals are subjected to an input process. First color information is obtained from the first RGB image signals. Second color information is obtained by a selective expansion processing in which color ranges except a first color range are moved in a feature space formed by the first color information, the first color information that represents each object in a body cavity being distributed in the each color range. The second color information is converted to second RGB signals. A red display signal, a green display signal and a blue display signal are obtained by applying a pseudo-color display process to the second RGB signals.