The physical picture for the melting of colloidal crystals is considered on basis of the Lindeman criterion. The existence of a minimal charge for crystallization of colloidal system (CS) is found. In general, this minimal charge Zmin is a function of the volume fraction of the macroions. For Z> Zmin there are restrictions for the existence of colloidal crystal for a region of not too high and not too low temperatures. This effect is connected with the main mechanism of crystallization, namely, with the strong interaction of overlapping Debye clouds, formed by counterions around the macroions. The results can be applicable also for dusty plasmas.