The research described in this article concerns the understanding of the additive colour process by children aged seven to eight years (N = 24) and the effectiveness of learning about this phenomenon while using a computer-based multimedia educational programme (MEP). First the children’s knowledge of the phenomenon was tested, then an intervention was organised for the experimental group during which they used the MEP, after which the children’s knowledge was tested again. Based on an analysis of the children’s conversations and their drawings, the way of understanding the phenomenon of additive colour was established. Three children’s conceptions of the understanding of the effect of the additive colour phenomenon are described (e.g., confusion with pigment mixing (RGB = CMY) and the claim that the effect of combining two additive colour creates a third colour (R + G = B)). Children’s behaviour during the use of the educational computer programme was also described and evaluated in terms of how close they were to the teaching strategy developed by the programme’s authors. The partial effectiveness of the MPE for the use of conclusions in a paper-and-pencil test was also investigated.