There is disclosed herein a display device comprising a picture generating unit, a waveguide pupil expander and a viewer-tracking system. The picture generating unit comprises a first display channel, a second display channel and a controller. The first display channel is arranged to output first spatially-modulated light of a first colour. The first spatially-modulated light corresponds to a first picture. The second display channel is arranged to output second spatially-modulated light of a second colour. The second spatially-modulated light corresponding to a second picture. The controller is arranged to drive the first display channel and second display channel. The waveguide pupil expander comprises a pair of parallel reflective surfaces. The waveguide pupil expander defines an input port and a viewing window. The input port is arranged to receive the first spatially-modulated light and the second spatially-modulated light. The viewing window is an area or volume within which a viewer may view the first picture and the second picture. The pair of parallel reflective surfaces is arranged to guide the first spatially-modulated light and the second spatially-modulated light from the input port to the viewing window by a series of internal reflections. The reflectivity of a first reflective surface of the pair of parallel reflective surfaces is provided by a graded coating. The graded coating is partially transmissive to light of the first colour and light of the second colour. The transmissivity of the graded coating is non-achromatic. The viewer-tracking system is arranged to determine a viewing position within the viewing window. The controller is arranged to maintain as substantially constant the colour balance of the first and second picture as seen from the viewing position based on the viewing position determined by the viewer-tracking system.