Diketopyrrolopyrroles are typical hydrogen-bonded pigments of red color on the basis of the skeleton of 1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP). Our previous investigation revealed that the 1:1 mixed crystal (MX-DPP) composed of DPP and its tert-butylphenyl derivative (BTB-DPP) gives practically the same electronic spectra as well as X-ray diffraction diagrams as those of the hybrid compound (MTB-DPP) of DPP and BTB-DPP. To elucidate the above coincidence, electronic structure has been studied in the present investigation in evaporated films and single crystals of MX-DPP and MTB-DPP with major focus on the crystal structure and intermolecular interactions. According to the structure analysis, the space groups of both crystals are the same (P $(1) over bar $) and the crystallographic parameters are also quite similar. In both crystals, the same molecular environment (in other words, "local similarity") prevails throughout the crystal as characterized by two pairs of "tert-butyl group and phenyl ring". These facts lead us to conclude that the local similarity is the determinant for the coincidence of the crystallographic and electronic structures.