1. In an ultrasound machine for generating an image responsive to moving structure within a region of interest of a subject by displaying at least one color characteristic corresponding to a movement parameter of said structure, apparatus for mapping said color characteristic comprising: a front-end arranged to transmit ultrasound waves into said structure and to generate received signals in response to ultrasound waves backscattered from said structure in said region of interest over a time period; a processor responsive to said received signals to generate a set of parameter signals representing values of said movement parameter within said structure during said time period and responsive to a distribution of said set of parameter signals and a mapping algorithm to generate a set of color characteristic signals representative of said values of said movement parameter; and a display arranged to display a color representation of said moving structure in response to said set of color characteristic signals.