A set of colours aesthetically pleasant are described as harmonious in the language of human visual perception. As this notion encloses a subjective part, a psychophysical experiment was carried out to estimate the perception of colour harmony for combinations of paintings with the uniform colour of walls on which they are hung.The experiment, that involved 38 observers, was based on accurate colours built upon a specific colour flow. Participants were asked to judge the colour harmony of combinations of a sample of 7 selected paintings with backgrounds uniformly coloured in 3 different ranges of colours – achromatic colours, tones derived from the global average colour of the considered painting and tones derived from the complementary of the global average colour of the considered painting.Results demonstrate that the best colour harmony is obtained when the average colour of paintings is used to colour their background. The experiment presented in this paper clearly shows that the white colour usually used for walls in museums does not optimize the colour harmony.