The art of pressed flowers is a method of artistic expression involving the pressing of flowers, leaves, and other plant organs for artistic creative purposes. However, the pressing process often results in color variation of the plant material, which significantly diminishes the quality of artistic works and must be solved using appropriate techniques. During this research, phenylalanine (10 mmol⋅L−1) was used to treat the petals of postharvest Petunia flowers to investigate the impact of phenylalanine on mitigating color variation, and the effect of phenylalanine on inhibiting the color variation of Petunia petals during the pressing process was evaluated by color measurement, physicochemical indices, and gene expression level analyses. Using the CIEL*a*b* color measurements, the samples from the test group had significantly higher brightness (L*) and red coloration (a*) at the final stage (S4) than the control group. In addition, phenylalanine had a significant inhibitory effect on malondialdehyde and superoxide anion accumulations in Petunia petals during pressing and reduced the enzymatic activities of superoxide dismutase, polyphenol oxidase, and catalase. The quantitative reverse-transcription polymerase chain reaction analysis showed that the transcript levels of CHS, DFR, F3′5′H, and UFGT genes in the petals of the treatment group continued to increase during the pressing process, and the transcript levels of key genes in the anthocyanin metabolic pathway of the treated samples were higher than those of the control group at the final stage (S4). These results indicated that phenylalanine can effectively diminish the color variation of Petunia petals in the pressing process, which could serve as a theoretical basis for the development of a comprehensive technology system aimed at preserving the color of pressed horticultural plants.