There is described an image processing method in which a scene is repeatedly imaged to form a series of input images. For at least a subset of the input images, a color calibration procedure is conducted which populates a foreground color histogram with the frequency of occurrence of color values in a stencil area of the input image, and populates a background color histogram with the frequency of occurrence of color values outside of the stencil portion of the input image. For at least a subset of the input images, a color replacement procedure is conducted which updates the stencil area based on a determination, from the color values of pixels within the input image, of likelihood values representing the likelihood of pixels belonging to an image area of interest, the likelihood value for each color value being determined from a combination of the foreground and background color histograms, replaces the original color values of pixels within the updated stencil area of the input image with replacement color values, and displays the image on which color replacement processing has been conducted. In this way, a stencil area is determined based on foreground/background histogramming, and used both to define an area to which color replacement processing is to be conducted, and also an area for use in further populating the color histograms to calibrate the color replacement processing.