Disclosed is a method of enhancing the visibility of blood vessels in a colour image captured by an image capturing device of a medical device. The colour image having a plurality of colour channels and a plurality of pixels. The method comprises for at least some of said plurality of pixels the steps of: (a) processing data obtained from a first colour channel together with data obtained from a second colour channel to determine a value of a first parameter indicative of the intensity in the red spectrum relative to the total intensity of said pixel； (b) using said value of said first parameter to alter said pixel, wherein said first parameter has at least three possible values, and wherein the strength of the alteration is dependent on the value of said first parameter.
