Since 1951, the Commission Internationale de L'Eclairage (CIE) has provided influential guidance on the choice of the most suitable colors for colored signal lights. In 1994, the CIE reviewed its 1975 recommendations for signal colors and has recently revised them. These revised recommendations have now been published as a CIE standard. This article reports an experiment designed to rest those recommendations and to provide data on the reliability of signal color recognition under a range of conditions. Thirty young subjects (aged 18-28 years) and thirty older subjects (aged 50-64 years) named the colors of lights, the colors of which were located on or close to the color boundaries defined by the CIE for red, yellow, white, green, and blue colors. The angular diameter of the light was one min of are. In a second experiment, half the subjects named the colors of the lights when the angular diameter was 5 min of arc. Observations were made under both dark and light adaptation. Red signals, especially those located in the CIE Class A domain, were reliably recognized under all conditions. Yellow near the red boundary of the CIE yellow domain tended to be confused with red, especially under dark adapted conditions at low signal illuminances. White was not a reliable signal color and was often confused with yellow, except for a white located near the blue boundary of CIE white. Green colors located near the blue boundary of the CIE color domain for green were less reliably recognized than those of longer dominant wavelength. Blue signal colors located within the CIE Class A blue domain were more reliable than expected, except at very low signal illuminances. There were significant differences in the performance of older compared to younger subjects, some of which can be explained by color shifts occurring as the result of the yellowing of the lens of the eye with age. (C) 2001 John Wiley & Sons, Inc.