The culture of microalgae is currently more and more developed at the industrial scale. In many applications, a substrate limitation is necessary to induce the production of a specific metabolite. The question of a better yield when inducing a stress by two nutrients simultaneously motivates a better modelling of microalgae colimited by two nutrients. We present a new model that represents growth of microalgae colimited both by nitrogen and phosphorus. We show that the key point in modeling this complex biological system is the expression for the absorption rate. Phosphorus uptake must be a decreasing function of the phosphorus internal quota. The situation for the nitrogen uptake is different and we show that it must be an increasing function of the phosphorus quota to be able to represent experimental observations. Finally we end up with a model that explains the a priori paradoxical opposite response of nitrogen and phosphorus to dilution rate. The proposed model is compared with data of Selenastrum minutum and is validated both qualitatively and quantitatively.