This paper presents an experimental study of the adsorption of colloids on model membranes mediated by specific ligand-receptor interactions. The colloids consist of lipid multilamellar liposomes (spherulites) functionalized with the B-subunit of Shiga Toxin (STxB), while the membranes are lipid Giant Unilamellar Vesicles (GUV) containing STxB lipid receptor, Globotriaosylceramide (Gb3). Through confocal microscopy and flow cytometry, we show the specificity of the adsorption. Moreover, we show that flow cytometry can be used to efficiently quantify the kinetics of colloid adsorption on GUVs with very good statistics. By varying the bulk colloid concentration and receptor density in the membrane, we point out the existence of an optimum Gb3 density for adsorption. We propose that this optimum corresponds to a transition between reversible colloid adsorption at low Gb3 density and irreversible adsorption, and likely spherulite fusion, at high density. We compare our results both to STxB-colloids adhering on living cells and to free STxB proteins interacting with GUVs containing Gb3. This biomimetic system could be used for a quantitative evaluation of the early stage of virus infection or drug delivery.