To elucidate the mechanisms of blue-white petal variation in Salvia farinacea, a comprehensive approach integrating phenotypic, cytological, biochemical and molecular perspectives was employed. The results indicated that blue flowers belong to the violet-blue group (N89A), whereas white flowers belong to the white group (NN155B). Compared with white-flowered plants, blue-flowered plants exhibit more abundant flowering, a longer flowering period, and stronger fragrance. Cytological observations revealed that the upper epidermal cells of blue flowers were conical and pigment-rich, whereas those of white flowers were spherical and colorless. Metabolic analysis showed that white flowers contained significantly lower anthocyanin levels but higher total flavonoid content, suggesting a metabolic shift toward the flavonoid pathway. Gene expression and cloning analyses further revealed that the white-flowered SfDFR (dihydroflavonol-4-reductase) harbored a double A-base insertion at nucleotides 159-160. This insertion caused a frameshift mutation, which probably give rise to premature translation termination, and it might be the determinant of the lack of delphinidin-based anthocyanins in S. farinacea. To our knowledge, this is the first time that the white-flowered phenotype might be caused by a double A-base insertion in the DFR. By integrating cellular, metabolic, and genetic analyses, this study deciphers the key differential mechanisms responsible for blue-white petal variation in S. farinacea. In summary, our results provide novel insights into the white color formation in higher plants, and provides key gene target and molecular marker for ornamental plant breeding.These findings hold potential for directional regulation of flower color and the development of novel ornamental varieties through technologies such as gene editing.