Luciferin used with luciferase results in:
Luciferase enzyme from fireflies and marine organisms catalyzes oxidation of substrate luciferin in ATP and oxygen dependent reaction. Energy released does not produce color change but photon emission in visible range, termed bioluminescence. No external excitation light is needed, unlike fluorescence of GFP which requires light source. Reaction produces low background and high sensitivity detection used in reporter assays, ATP quantification and immunoassays. Green fluorescence would require fluorophore excitation, while luciferin luciferase system generates intrinsic light production measurable by luminometer for quantitative biology.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.