Cutting speed is a key factor that influences machined surface quality and tool life in milling. To date, the study of its distribution over a machined surface has not been established. This paper presents a mathematical model to evaluate cutting speed on the machined surface in 3-axis ball nose milling. The approximation errors introduced by surface shape, step-over and CNC interpolation are analysed. The model is used to predict visible areas which have different colour intensities on finished surfaces in the machining of a wood-plastic composite. A good agreement is obtained between the prediction and experimental results.