1. Color and brightness tracking control apparatus for a color cathode ray tube display instrument system subjected to viewing under a wide range of ambient light conditions comprising (a) a cathode ray tube having a display screen for emitting images in a plurality of different colors dependent upon the independent and variable energization of cathode means for producing at least two independent primary colors the relative brightnesses of which determine said plurality of colors, (b) video command means for commanding at least one image to be displayed in at least one predetermined color comprised of components of said two primary colors at the required relative brightness levels, (c) ambient light sensor means for providing a signal corresponding to the range between the extremes of ambient light conditions existing in the vicinity of said display instrument, (d) computer means including (i) memory means containing data representing the independent cathode energizations required to produce each of said primary color component relative brightnesses over said range of ambient light conditions, and (ii) processor means responsive at least in part to said light sensor means for continuously computing a reference display brightness and for deriving from said memory means cathode energization data required to produce said two primary color component relative brightnesses at the existing ambient light conditions, and (e) means responsive to said video command means and said derived cathode energization data for energizing said cathode means to thereby produce said predetermined color image at the existing ambient light conditions.