Self-assembled photonic structures have been under theoretical and experimental study for decades, whereas previous theories on optical properties were mainly concerned with perfect structure or some certain limited kinds of disordered photonic crystals (PCs), making them unsuitable for characterizing the real self-assembled PCs. In order to improve our understanding of the mechanism of self-assembly and provide more crucial clues to further grow perfect crystals, we extended previous widely used scalar wave approximation (SWA), making it be able to characterize long-range disorder (β) and short-range disorder (α) in PCs synthetically in a simple and effective way. Excellent agreement with in situ observed reflectance of evaporation induced self-assembled colloidal photonic crystals (CPCs) was obtained, demonstrating that the introduction of the parameters α and β in SWA can successfully characterize the disorder in self-assembled CPCs. Furthermore, extended SWA was further used to study the disorder formation in self-assembly, and it was found that during growing stage both β and α drop down, whereas in drying stage β stays nearly unchanged while α increases significantly. It turned out that the growing stage of self-assembly is a stage when the structure transforms from disordered to ordered one, and growth induced disorder mainly arises in drying stage. The results obtained provide an insight into the growth mechanisms of self-assembly and theoretical basis for characterizing optical properties of disordered PCs.