Abstract: The colour palette of painters over history has been of interest to many, including: art historians, archaeologists, and art lovers. Colour usage in art changes from culture to culture and season to season and is often thought of as reflecting or inspiring mood and ambience. We present ColourVis: a visualisation that supports exploration of colour usage in digital images. In particular, we use as a case study European art over the last six centuries. Visualising this relatively unexplored area offers insights into such questions as: How blue was Picasso's blue period?; How do realist painters' colour choices compare to that of surrealist painters; or How has the usage of colours changed over time? Through ColourVis we offer an exploration and comparison tool for individual paintings, groups of paintings and trends in colour usage over time.