In the production of tomatoes in greenhouses, a more or less constant number of tomatoes are ready for picking at each harvest time. In the production of bell peppers, however, large fluctuations exist in number of fruits ready to pick. This major problem in bell pepper production is called `flushing¿: weeks with high yields are alternated by weeks with low yield. This irregular harvest pattern makes it difficult for growers to meet regular weekly demands. As this cyclic production pattern is more or less the same for all growers, it results in weeks with a high market supply and low prices alternating with weeks with a low market supply and high prices. Flushing also results in strong fluctuations in labour demand in the greenhouses. Hence, avoiding an irregular pattern of sweet pepper production is of great economic importance. Flushing makes it very difficult for the growers to allocate sufficient labour force at a certain time in production. Both allocating too many and too few pickers will have financial consequences for the grower. The aim of this study is to predict the optimal harvest date based on repeated colour measurements of the growing bell peppers. The fruit ripening of bell peppers has been followed by colour imaging in the greenhouse. A commercial digital camera was used to record the development of colour for two cultivars of bell peppers over time. The images were analysed by image processing to obtain the R, G and B values. At the same time the time to harvest was recorded for the same fruits. Using these data a model was developed, on the basis of an exponential function towards a fixed asymptotic end value, to describe the colour development during fruit growth in terms of R/G ratio and to predict the optimal harvest date of the fruits.