Eleven observers made colour matches between LCD and CRT monitors and paint samples in viewing conditions similar to those of soft-proofing. The matches were used to evaluate the practical significance of observer metamerism and of failures of colorimetric additivity in cross-media colour matching.The individual variations in matches are of magnitudes that are expected to have practical consequences in graphic arts applications; they can not be explained by observer metamerism and thus can not be modelled by the Standard Deviate Observer. At the other hand, these variations are modelled well by the CIEDE2000 colour difference formula.Failures of colorimetric additivity lead to systematic disagreements in cross-media matches made by individual observers and predicted by the CIE 1964 Standard Colorimetric Observer. The discrepancies are consistent with all the reports on the subject, but have never been confirmed to exist in practical colorimetry. A chromatic adaptation modelling framework can be used to compensate for the failures in practical applications.We conclude that additivity failure is a significant contributor to the uncertainty of colour matching, and needs to be accounted for in industrial colour management systems. The practical implications of individual variability which is not the result of observer metamerism remains unclear.