The darkening of lead chromate yellow pigments, caused by a reduction of the chromate ions to Cr(III)-compounds, is known to affect the appearance of several paintings by Vincent van Gogh. In previous papers of this series, we demonstrated that the darkening is activated by light and depends on the chemical composition and crystalline structure of the pigments. In this work, the results of Part 2 are extended and complemented with a new study aimed at deepen knowledge on the nature and distribution of Cr and S-species at the interface between the chrome yellow paint and the non-original coating layer. For this purpose three micro-samples from two varnished paintings by Van Gogh and a waxed low relief by Gauguin (all originally uncoated) have been examined. Since non-original coatings are often present in artworks by Van Gogh and contemporaries, the understanding of whether or not their application has influenced the morphological and/or physico-chemical properties of the chrome yellow-paint underneath is relevant in view of the conservation of these masterpieces. In all the samples studied, microscopic X-ray fluorescence (µ-XRF) and X-ray Absorption Near Edge Structure (µ-XANES) investigations showed that Cr(III)-based alteration products are present in the form of grains inside the coating (generally enriched of S-species) and also homogeneously widespread at the paint surface. The distribution of Cr(III)-species may be explained by the mechanical friction caused by the coating application by brush that picked up and redistributed the superficial Cr-compounds, likely already present in the reduced state as result of the photo-degradation process. The analysis of the XANES profiles allowed us to obtain new insights into the nature of the Cr(III) alteration products, that were identified as sulfate-, oxide-, organo-metal- and chloride-based compounds. Building upon the knowledge acquired through the examination of original paint-samples and from the investigation of aged model paints in the last Part 4 paper, in this study we aim to characterize a possible relation between the chemical composition of the coating and the chrome yellow degradation pathways by studying photochemically aged model samples covered with a dammar varnish contaminated with sulfide and sulfate salts. Cr-speciation results did not show any evidence of the active role of the varnish and added S-species on the reduction process of chrome yellows.