A number of general ergonomic rules pertaining to absolute discrimination indicate that for recognising and naming a colour, a maximum of about seven colours is desirable. When it comes to seeing the difference between colours, many different colours and shades can be distinguished, probably well over twenty. Research on colour screens has shown, however, that this is unrealistic. One cannot normally have more than three to seven colours on a CRT, depending on a number of different factors (Kinney, 1979; Teichner, 1979; Silverstein and Maryfield, 1981). For an operational colour display where absolute colour discrimination is required, it is suitable to have three to four colours, and these should preferably be green, red, blue-green, and possibly a purple-red (plus white or black, depending on the background colour used). Where comparison between colours is possible, six to seven colours can be allowed, preferably red, yellow, green, blue-green (cyan), reddish-blue, and perhaps magenta (plus white or black depending on the background).