Symbols should be legible and readily discriminable against the background colors under all expected ambient lighting conditions.
Additional Information:
For adequate legibility, colored symbols should differ from their color background by an E distance (CIE Yu'v') of 100 units or more. The E distances (CIE Yu'v') are derived from the 1976 CIE UCS color diagram. As with the (CIE L*u*v) distances, caution should be used in assessing legibility for characters in colors having small luminance differences. This caution applies not only to characters in color but also to small luminance differences in background colors and for very small luminance differences between characters in color and background in color. Unusually large or small characters may lead to erroneous estimates of legibility. The elements required for the calculation are the luminance in cd/mē (Y) and the UCS coordinates (u',v') of the text and background. The metric is given in Equation 1.2.