The article investigates five types of parametric effects in assessing colour difference. The visual assessment experiments were conducted in six phases. The viewing parameters studied include the sample separation, sample size, and the background colours. The dataset prepared in this study, includes a total of 107 cotton-dyed pairs belonging to five colour centers. The average colour difference of the sample pairs was about 5.0 CIELAB units. Gray-scale method was used fbr colour-difference assessment with a panel of 10 observers (2 assessments each). The CIELAB colour-difference formula was found to have the best performance over other formulae using the current dataset. In general, the parametric effects vary from 8-15%. When,further analyzing the colour centers, it was found that much larger parametric effects of 42% and 26% were associated, respectively, with green sample pairs assessed on the green background and blue samples pairs assessed on the blue background. The green and blue background colours in these two cases are very close to the green and blue colour centers selected, which clearly indicates the crispening effect.