A two stage method for allowing direct perfect superimposition and comparison of Fresnel-transform reconstructions of digital holograms recorded at different wavelengths is proposed and demonstrated. The method allows to adjust the size of the reconstruction pixel by varying the reconstruction distance of the first stage. Demonstration is given by superimposing in focus numerical reconstructions of holograms recorded at different wavelengths. The method can be potentially very useful for real-time monitoring in biological processes.