A large set of colour discrimination data based upon Cathode Ray Tube (CRT) colours was accumulated. The experiment was carried out to study the effect changes in viewing parameters on perceived colour differences. It was divided into sixteen phases. Each phase was conducted under a different set of experimental conditions including different sample sizes, backgrounds, frames, widths. and colours of separation between pairs of samples. For each phase, 134 pairs of colours with average 2.7 CIELAB colour difference were studied The colours closely corresponded to the five colour centers proposed by the CIE. Each pair was assessed by, 10 or 20 observers using the gray, scale method. The visual results from the present study were also used to test six colour-difference formulae: CIELAB, CMC, BFD, CIE94, LCD, and CIEDE2000. The latter outperformed the other formulae and is the new CIE colour-difference equation. The current results were also compared with the other datasets based upon surface colours. The present results are consistent with those obtained for surface colours.