ColorReplace

ColorReplace[image, color]
finds regions in image whose pixel values are similar to color and replaces them with transparent pixels.

ColorReplace[image, color->replacement]
replaces all pixels with the specified replacement color.

ColorReplace[image, color, t]
replaces all pixels whose values are within a distance t from color.

ColorReplace[image, marker]
uses the mean of the pixel values in image specified by the marker image as a reference color.