The covalent immobilization of synthetic or natural macromolecular compounds containing amino groups onto polystyrene (PS) solid surfaces is of great interest in diagnostic applications. A sensitive assay allowing the determination of reactive end groups is therefore a powerful tool for predicting the performance of the active surface. Recently, we reported the use of the Coomassie Brilliant Blue (CBB) colorimetric reagent to quantify protonated groups (N+) in linear and dendritic structures in solution (Coussot G et al. Polym Int 58(5):511-518). In this work, a simple method using CBB dye for the characterization of PS aminated solid-surfaces is developed. The proposed Amino Density Estimation by Colorimetric Assay (ADECA) method is based on the reversible complexation of the dye with the N+ groups on solid surfaces. The assay measures the released dye thanks to the use of a unique sodium carbonate-methanol buffer. Thereby, for the first time, the same surface can be used for characterization and for further coupling applications. A surface density of 4 N + groups per nm2 can be measured in PS microwell format, the whole characterization being done within 30 min. Performances of this new colorimetric-based method are detailed. The ADECA method is further demonstrated to be useful for the characterization of aminated polypropylene, and glass materials with various sizes and shapes.