This paper surveys some major factors which commonly affect colour in paintings over time, with particular reference to art historical judgements about artists' original intentions. A range of the most common causes of colour alteration, both external and intrinsic, such as surface dirt, darkening varnish, over-painting and old re-touching, the natural yellowing of ageing oil, the rising refractive index of drying oil, fugitive pigments and the effects of light is considered. The extent to

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... these changes in the appearance of paintings have consequences for taste, have implications for the training of art historians and affect how far conservators can, or should, restore colours to their original state is briefl y noted.