Plant anthocyanin biosynthesis is regulated by a combination of transcription factors. Here, exogenous anthocyanin regulator genes Delila/Rosea1 (Del/Ros1) or Bperu/Colored aleurone1 (B1/C1) were introduced to Lilium ‘Sorbonne’ using an efficient and optimized Agrobacterium tumefaciens (Smith & Towns.) Conn-mediated transformation system. During the B1/C1 transformation, eight types of modified Murashige and Skoog medium for bacterial inoculation and co-cultivation were evaluated. Significant enhancement in putative transformation efficiency was observed upon the removal of KH₂PO₄, NH₄NO₃, and KNO₃ or with the removal of all the macroelements. Using the optimal medium, 20 s sonication positively affected the transformation while no significant improvement was observed following heat shock treatments. Replacing sucrose in the co-cultivation medium with high concentrations of maltose or glucose also promoted the transformation. Higher transformation efficiencies were observed when using this optimized protocol for the Del/Ros1 transformation. The Del/Ros1 and B1/C1 transformed plantlets appeared light purple during the preliminary regeneration stage, with some deeper purple tissues also observed in Del/Ros1 plantlets. After the transformation was confirmed by polymerase chain reaction and Southern blot, the colour unexpectedly faded to normal green when the transgenic plantlets were rooted, but significant elevations in anthocyanin levels remained in leaves and scales compared with the nontransgenic plantlets. The introduction of these anthocyanin regulator genes may be useful for the modification of Lilium flower colour.