This Technical Report reviews the CIE international recommendations for the colours of signal lights. The CIE recommendations for signal colours were first adopted in 1951 and were revised in 1975. They have not since been subject to critical review in the light of new experimental evidence and changed technology. Published and unpublished reports of experiments on the recognition of signal colours were compiled and analysed. The data from those reports were brought on to a common base for dim signals, moderately bright signals and bright signals under conditions simulating both night and day observation. Some of the experiments included subjects with defective colour vision. Consideration of the data by the Committee and its expert advisors has lead to a number of recommendations for changes to the internationally accepted colour domains for signal colours.