In horticulture and floriculture, plants are predominantly propagated vegetatively from stem cuttings. Consequently, the success of propagation depends on a plant’s capacity to form adventitious roots (AR) at the basal part of the cutting, and AR formation depends on the interaction between flavonoids and plant hormones in the basal part. In ornamental plants, flavonoid accumulation is important for flower color and can interact with plant hormone activity. In this study, we used an aeroponic system to culture two Campanula portenschlagiana genotypes contrasting in flower color to estimate cultivar differences in AR formation, flavonoid concentrations, and hormone composition. We found a greater accumulation of flavonols and flavones in petals in a blue-flowered genotype than in a white-flowered genotype. The blue-flowered genotype also accumulated more flavonols and flavones in the basal part of cuttings and showed better AR formation. The better AR formation in the blue-colored genotype was related to a greater accumulation of auxin and a lower accumulation of ABA in the basal part of the cuttings. In conclusion, higher flavonoid levels in the basal part of cuttings may interact with hormone action to affect excision-induced AR formation during the vegetative propagation of Campanula.