This study investigated the differences between different large colour-difference (LCD) data sets (with a mean Delta E*(ab) value about 10). Six data sets were studied. For each data set, various CIELAB based colour difference models were derived to fit the data. These models were compared to shed light on the difference between the different data sets. It was found that all data sets have very similar characteristics except for the Munsell data. Detailed investigation showed that the discrepancy is mainly due to the balance between the lightness and chromatic differences used previously for the Munsell data set. It was found that one unit of Munsell Value appears to be three times as large colour difference as one unit of Munsell Chroma at least under the experimental conditions for the data sets studied here. (c) 2010 Wiley Periodicals, Inc. Col Res Appl, 36, 111 - 117, 2011; Published online 24 February 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/col.20591