Researchers in social sciences and humanities disciplines are confronted with the need to analyse increasing amounts of visual data, which calls for the development of new computational methods. This paper focuses on the problem of identifying the colours used in children's drawings, notably in the perspective of assessing their diversity. It describes a simple, effective, and flexible algorithm for performing this task. This methodology is applied to a subset of more than 1000 drawings taken from the ``Drawings of gods'' database. The first results show that this approach makes it possible to address meaningful research questions concerning the patterns of colour usage in these data.