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This work discusses the curing-induced discolouration of UV varnishes, the so-called ‘yellowing’. A study of 47 market available acrylic-based UV varnishes from five manufacturers shows that all varnishes exhibit significant yellowing. In order to clarify if yellowing can be exploited for monitoring the cure of the varnishes, three varnishes are analysed in detail, including one low-migration and one photoinitiator-free self-initiating formulation. For this purpose, the applied varnishes are measured with respect to yellowing as well as their actual polymerisation degrees. Both measures are checked for correlations to develop a novel method for monitoring the degree of polymerisation based on colourimetric measurements. Additionally, the temporal behaviour of the yellowing is determined as it is of significant importance when considering a practical usage of the yellowing approach.