The colour of an observed object can be described in many different manners, and the description by its reflectance provides the unambiguous colour representation. The reflectance description can be acquired by expensive multispectral cameras or, e.g., with time-sequential multispectral illumination. In our experiment, we propose that under the condition of constant and uniform illumination, the reflectance can be deduced from the object's RGB camera readouts, captured alongside the set of colour patches with known spectral characteristics. Translation from a colour description in RGB space into reflectance spectra, independent of illuminant and camera sensor characteristics, was performed with the help of an artificial neural network (ANN). In our study, the hypothesis was proposed that the ANN's performance of reflectance reconstruction can be enhanced by employing richer learning datasets using RGB input sets of two cameras instead of just one. Additional second camera information would be adequate only if the equivalent channels of cameras used are linearly independent. A quantitative measure of nonlinearity (QMoN), which is the metric primarily developed for use in chemistry, was employed to estimate the degree of nonlinearity. Additional attention was paid to ANN training, structure and learning set sizes. Two ANN training algorithms have been utilised, a faster GPU executed standard backpropagation and an order of magnitude slower CPU based, but with significantly better convergence Levenberg-Marquardt training algorithm. The number of neurons in the hidden ANN layer varied from the size of the input layer to a number greater than the number in the output layer. The complete set of colour samples was divided into five learning sets of different sizes, with the smaller sets being subsets of the larger ones. To assess performances of the resulting ANNs, mean squared error, the goodness of fit and colour differences calculated from original and reconstructed reflectances assuming several standard illuminations have been compared. A noticeable reflectance performance improvement has been found by using two cameras, even though the cameras' equivalent channels exerted only small degrees of nonlinearity.