Colour is an important and useful feature for object tracking and recognition but it has the difficulty that the colour of the object changes if the illuminant colour changes. However, if the illuminant colour is known it becomes a robust feature. There are more and more computer vision applications containing humans, for example in interfaces for human computer interaction or automatic camera men, where skin colour is an often used feature. Hence, it would be of significant importance to know the illuminant in such applications. This paper proposes a novel method to estimate the current illuminant colour from skin colour observations. The method is based on a physical reflection model, the assumption that illuminant colours are located close to the Planckian locus, and the knowledge about the camera parameters. The method is empirically tested using real images. The average estimation error is as small as 180 Kelvin. Applications are for example in colour based tracking and in visualisation to re-render image colours to their appearance under canonical viewing conditions.