Red and blue lights are important for plant growth, anthocyanin production and photosynthesis, as they are absorbed by chlorophyll and trigger various physiological processes, including photomorphogenesis and phototropism, ultimately influence plant development and morphology. The objective of this study was to examine the effects of combined light spectra to enhance plant growth and anthocyanin concentration of red leaf lettuce in a plant factory. In this experiment, five treatments were set up by adjusting the relative amount of red (peak wavelength of 660 nm) and blue light (peak wavelength of 450 nm) ratios of 6:1, 3:1, 1:1, 1:3, and 1:6, which were recorded as R 6B 1, R 3B 1, R 1B 1, R 1B 3, and R 1B 6, with a total photosynthetic photon flux density of 200 μmol m −2s −1. Lettuce ( Lactuca sativa cv. ‘Lolla Rossa’) plant biometric, morphological, and nutritional quality parameters were investigated. On day 27 after transplanting, the results showed that: (1) fresh and dry plant weights, leaf length and leaf area increased by 172.7%, 105.8%, 55.4% and 121.4%, respectively, under treatment R 6B 1, as compared with those under treatment R 1B 6. (2) no significant difference was observed under all treatments in stem length, photosynthetic rate, and stomatal conductance. Water use efficiency was the highest under treatment R 6B 1, with a 59.9% increase over that under treatment R 1B 6. On contrary to this, (3) both anthocyanin and nitrate concentrations increased by 155.5% and 58.3%, respectively, under treatment R 1B 6 than treatment R 6B 1. While sugar concentration favoured a higher proportion of red-light ratio. (4) carotenoids concentration was the highest under treatment R 1B 1 with an increase of 140.1% as compared with that under treatment R 1B 6. The above results indicate that more red light contribute to increase plant biomass, while elevating amount of blue light leads to enhancing anthocyanin production and nutritional quality in lettuce plants. R 1B 1 can be considered as a balanced ratio of red and blue lights for promoting photosynthate accumulation and desirable nutritional quality, including anthocyanin concentration per red leaf lettuce plant. Further research may elucidate the underlying mechanisms driving these responses and refine lighting strategies for optimizing lettuce production and nutritional quality.