Colloidal crystals of mono-dispersed polystyrene spheres in aqueous solvent were formed by a new drop-slide method. The optical properties of the colloidal crystal array (CCA) were characterized by reflection spectroscopy. The very strong intensity of reflectance was obtained by the new method. The maximum of the diffraction peak is highly symmetric and has a relatively narrow line width of about 52.0 nm when the CCA is in the wet state. An in-situ real time investigation on the formation of the CCA was performed. The reflection maximum was found to be 566.4nm, which blue shifted to 486.5 nm over the first drying phase and finally to a minimum of 432.3nm upon reaching the final stable state. The shifts of peak position were determined to be 4.1 nm / min at the first drying phase. The analysis of the diffraction maximum with calculation shows that the colloidal particles can crystallize into a face-centered-cubic (fcc) structure. The scanning electron micrograph image of the polystyrene spheres after is completely dried shows a face centered cubic formation.