Light spectrum is a crucial environmental factor influencing plant growth and secondary metabolite production in controlled-environment agriculture. This study investigated the combined effects of light spectral composition on growth performance and andrographolide accumulation in Andrographis paniculata (Burm.f.) cultivated in a plant factory system. Two Thai cultivars, RBR and TTT, were grown under white light and various red–green–blue (R:G:B) LED ratios during the vegetative and flowering stages. Plant morphological traits, biomass accumulation, and andrographolide derivatives (AP1, AP4, AP6, and total AP) were quantified. Growth and biomass production were significantly enhanced under white light and red-enriched spectra, particularly during the vegetative stage, whereas stem elongation exhibited reduced sensitivity to light spectral quality during the flowering stage. In contrast, andrographolide accumulation showed a strong cultivar-dependent response to light spectrum. The TTT cultivar showed pronounced increases in AP1, AP4, AP6, and total AP under blue-enriched spectra, with the 60B:10G:30R treatment producing the highest total diterpenoid content in both cultivars, while the RBR cultivar displayed limited responsiveness to spectral variation. These results demonstrate light spectrum affecting both biomass production and phytochemical accumulation. Optimization of light spectral composition combined with appropriate cultivar selection offers an effective strategy for enhancing pharmaceutical-grade andrographolide production in plant factory systems.