A display comprising at least two cavity structures 40 substantially juxtaposed, the interiors of the cavity structures 40 being respectively coloured, at least in part, with a test colour and a reference colour, wherein the test colour is related to the reference colour in that it has a substantially similar or the same hue characteristic, but a different lightness characteristic. The cavity structures 40 may be boxes, paint cans, miniature rooms or full-sized rooms. The interior of the cavity structures 40 may be viewed through apertures 46 and may be illuminated by lamps 27. The two cavity structures 40 preferably have the same size and configuration and may have objects inside of them arranged in the same manner. The interiors of the cavity structures 40 may be painted or be fitted with coloured inserts to display the test and reference colours. The test and reference colours may have the same or similar chroma characteristics and the reference colour may be selected from a colour-difference algorithm, possibly a CIE algorithm in dependence on the characteristics of the test colour, using higher weighting values for hue and chroma values. A second invention defines a test determining viewer perception of cavity size using the display, with the lighter test colour supposedly making the cavity look bigger.