Some of Morpho butterfly species have a mysterious physical coloration. Their blue color has both high reflectivity (&gt
60%) and a single color in too wide angular range (&gt
± 40° from the normal), which are contradicting with each other from viewpoint of the optical interference. A key to the mechanism of the specific Morpho-color was suggested to be the nano-randomness in arrangement of the nanostructures on its scale, which prevents the rainbow interference. However, concrete optical roles of the nano-randomness remained still unclear. Using finite-difference time-domain (FDTD) analysis, we have recently investigated the optical role of different kinds of randomness in the nanostructure on the Morpho butterfly's scale. The results revealed clearly several independent roles of different kinds of randomness. On the other hand, by inproving the accuracy of simulation, we have found new aspects on the analysis, especially for the number of random components (nano-trees). These new aspects will give important hint and caution to futher simulation on the optical properties of this specific colorations that have wide potential applications. The direction obtained by the numerical simulations to analyze optically complex random structures will serve not only to understand the scientific principles, but also to design the optical properties of artificial materials. © 2013 SPIE.