Prior to the introduction of numerical evaluation of colours and equipment to measure their values optically, it was impossible to objectively estimate colour deviations resulting from changed yarn count or changed warp- and weft-thread density. Even today, it is practically impossible to carry out a systematic research of this influence on real fabrics, because it is impossible to produce such a large number of referential patterns. CAD systems enable relatively quick and cost-effective production of simulations of multicolour patterns. They even enable certain parameters to be changed within ranges which can be hardly achieved, or not at all, in practice. By the measurements carried out on these simulations, colour deviations due to the changed yarn count or warp and weft threads density can be estimated at a wider interval. However, the aim of the research is to determine the principles of forming colour deviations, and above all to estimate quantitatively the influence of the changed yarn count and thread density on the colour values of a fabric simulation. The paper presents the simulations of a defined number of fabrics made of yarns with varying counts and densities, the results of the colour measurements, and the analysis of the colour differences.