1. An ultrasound imaging apparatus, comprising: an ultrasound probe configured to transmit ultrasound signals and to receive ultrasound echo signals; an ultrasound system configured to process the ultrasound echo signals to yield a two-dimensional ultrasound image representing an image plane within a subject; a display connected to the ultrasound system and the display displays the ultrasound image; a position-sensing system configured to monitor a position and orientation of the ultrasound probe in a coordinate system to yield probe position and orientation information; a processor configured to: process the probe position and orientation information to determine: a display location in the ultrasound image corresponding to a perpendicular projection of a predetermined location onto the image plane and a distance between the predetermined location and the image plane within a slab of a thickness defined by a positive slab surface and a negative slab surface which are a positive and negative distance from the image plane respectively; and superimpose on the ultrasound image a display location marking indicia at the display location, and the indicia having one or more appearance characteristics determined at least in part by the distance between the predetermined location and the image plane within the slab, and the one or more appearance characteristics include a hue which is a continuous function of three colors within the slab.